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

Diversity of Intercellular Communication Modes: A Cancer Biology Perspective

From the moment a cell is on the path to malignant transformation, its interaction with other cells from the microenvironment becomes altered. The flow of molecular information is at the heart of the cellular and systemic fate in tumors, and various processes participate in conveying key molecular information from or to certain cancer cells. For instance, the loss of tight junction molecules is part of the signal sent to cancer cells so that they are no longer bound to the primary tumors and are thus free to travel and metastasize. Upon the targeting of a single cell by a therapeutic drug, gap junctions are able to communicate death information to by-standing cells. The discovery of the importance of novel modes of cell-cell communication such as different types of extracellular vesicles or tunneling nanotubes is changing the way scientists look at these processes. However, are they all actively involved in different contexts at the same time or are they recruited to fulfill specific tasks? What does the multiplicity of modes mean for the overall progression of the disease? Here, we extend an open invitation to think about the overall significance of these questions, rather than engage in an elusive attempt at a systematic repertory of the mechanisms at play.

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.

Long-range gap junctional signaling controls oncogene-mediated tumorigenesis in Xenopus laevis embryos

In addition to the immediate microenvironment, long-range signaling may be an important component of cancer. Molecular-genetic analyses have implicated gap junctions-key mediators of cell-cell communication-in carcinogenesis. We recently showed that the resting voltage potential of distant cell groups is a key determinant of metastatic transformation and tumor induction. Here, we show in the Xenopus laevis model that gap junctional communication (GJC) is a modulator of the long-range bioelectric signaling that regulates tumor formation. Genetic disruption of GJC taking place within tumors, within remote host tissues, or between the host and tumors significantly lowers the incidence of tumors induced by KRAS mutations. The most pronounced suppression of tumor incidence was observed upon GJC disruption taking place farther away from oncogene-expressing cells, revealing a role for GJC in distant cells in the control of tumor growth. In contrast, enhanced GJC communication through the overexpression of wild-type connexin Cx26 increased tumor incidence. Our data confirm a role for GJC in tumorigenesis, and reveal that this effect is non-local. Based on these results and on published data on movement of ions through GJs, we present a quantitative model linking the GJC coupling and bioelectrical state of cells to the ability of oncogenes to initiate tumorigenesis. When integrated with data on endogenous bioelectric signaling during left-right patterning, the model predicts differential tumor incidence outcomes depending on the spatial configurations of gap junction paths relative to tumor location and major anatomical body axes. Testing these predictions, we found that the strongest influence of GJ modulation on tumor suppression by hyperpolarization occurred along the embryonic left-right axis. Together, these data reveal new, long-range aspects of cancer control by the host's physiological parameters.

A novel method for inducing nerve growth via modulation of host resting potential: gap junction-mediated and serotonergic signaling mechanisms

A major goal of regenerative medicine is to restore the function of damaged or missing organs through the implantation of bioengineered or donor-derived components. It is necessary to understand the signals and cues necessary for implanted structures to innervate the host, as organs devoid of neural connections provide little benefit to the patient. While developmental studies have identified neuronal pathfinding molecules required for proper patterning during embryogenesis, strategies to initiate innervation in structures transplanted at later times or alternate locations remain limited. Recent work has identified membrane resting potential of nerves as a key regulator of growth cone extension or arrest. Here, we identify a novel role of bioelectricity in the generation of axon guidance cues, showing that neurons read the electric topography of surrounding cells, and demonstrate these cues can be leveraged to initiate sensory organ transplant innervation. Grafts of fluorescently labeled embryological eye primordia were used to produce ectopic eyes in Xenopus laevis tadpoles. Depolarization of host tissues through anion channel activation or other means led to a striking hyperinnervation of the body by these ectopic eyes. A screen of possible transduction mechanisms identified serotonergic signaling to be essential for hyperinnervation to occur, and our molecular data suggest a possible model of bioelectrical control of the distribution of neurotransmitters that guides nerve growth. Together, these results identify the molecular components of bioelectrical signaling among cells that regulates axon guidance, and suggest novel biomedical and bioengineering strategies for triggering neuronal outgrowth using ion channel drugs already approved for human use.

Connexins in migration during development and cancer

Connexins, the gap junction proteins, through their multitude of actions are implicated in a variety of cell processes during animal development and cancer. They allow direct or paracrine/autocrine cell communication through their channel and hemi-channel functions. They enable adhesion and interact with a plethora of signalling molecules. Here, we review the common themes in developmental and pathological processes and we focus in their involvement in cell migration in four different systems: neurons, astrocytes, neural crest and cancer.

Connexin26-mediated transfer of laterality cues in Xenopus

A cilia-driven leftward flow of extracellular fluid breaks bilateral symmetry in the dorsal midline of the neurula stage vertebrate embryo. The left-specific Nodal signaling cascade in the lateral plate mesoderm (LPM) is key to asymmetric morphogenesis and placement of organs during subsequent development. The nature of the initial asymmetric cue(s) as well as the transfer of information from the midline to the left side has remained elusive. Gap junctional communication has been previously involved in Xenopus left-right (LR) development, however a function at cleavage stages was inferred from inhibitor experiments. Here we show by heptanol-mediated block of connexin function that flow stages during neurulation represent the critical time window. Flow in Xenopus occurs at the gastrocoel roof plate (GRP), a ciliated sheath of cells of mesodermal fate transiently positioned within the dorsal epithelial lining of the forming archenteron. We reasoned that endodermal cells immediately adjacent to the GRP are important for transfer of asymmetry. A systematic screen identified two connexin genes, Cx26 and Cx32, which were co-expressed in these lateral endodermal cells. Gain- and loss-of-function experiments pinpointed Cx26 as the critical connexin for LR development, while Cx32 had no effect on laterality. Importantly, GRP morphology, ciliation and flow were not affected in Cx26 morphants. Our results demonstrate a decisive role of Cx26 in the transfer of laterality cues from the GRP to the left LPM, providing a novel access to the identification of the initial asymmetric signal generated by flow.

Connexin 33 impairs gap junction functionality by accelerating connexin 43 gap junction plaque endocytosis

Connexin 33 (Cx33) is a testis-specific gap junction protein. We previously reported that Cx33 exerts dominant-negative effect on gap junction intercellular communication by sequestering Cx43 within early endosomes in Sertoli cells. However, the molecular mechanisms that drive this process are unknown. The present study analyzed: (i) the trafficking of Cx33 and Cx43 in wild-type Sertoli cells transfected with Cx33-DsRed2 and Cx43-green fluorescent protein vectors; (ii) the formation of heteromeric Cx33/Cx43 hemi-channels and their incorporation into gap junction plaques. Fluorescence lifetime imaging microscopy-fluorescence resonance energy transfer and videomicroscopy studies demonstrated that Cx33 and Cx43 associated to form heteromeric oligomers that trafficked along microtubules to the plasma membrane. However, the plaques containing Cx33 were not functional. Immunoprecipitation experiments revealed that zonula occludens-1 (ZO-1), a scaffold protein proposed to secure Cx in gap junction plaques at the cell-cell boundary, associated with Cx33 in testis extracts. In cells expressing Cx33, Cx33 and ZO-1 specifically interacted with P(1) phosphorylated and P(0) unphosphorylated isoforms of Cx43, and the ZO-1 membranous signal level was reduced. It is suggested that alteration of Cx43/ZO-1 association by Cx33 could be one mechanism by which Cx33 exerts its dominant-negative effect on gap junction plaque.

Dynamic ATP signalling and neural development

Purinergic signalling plays a major role in the function of every organ including the brain. A growing body of evidence also suggests that purinergic signalling is important in the development of the retina, cochlea and neocortex. In these three contexts release of ATP through the spontaneous gating of connexin hemichannels in cells, respectively, of the retinal pigment epithelium, Köllicker's organ, and the radial glia triggers waves of intracellular Ca(2+) release. In the case of the developing retina and cortex, the released ATP acts to control proliferation of neuronal precursor cells, while in the cochlea it coordinates the spontaneous activity of adjacent hair cells to refine the tonotopic maps in the cochlear nucleus. Recently ATP-derived ADP signalling has been implicated at the very earliest stages of development, notably in triggering the gene expression necessary for formation of the eye. It is now timely to test the extent to which connexin hemichannel-mediated ATP release and accompanying Ca(2+) waves contribute to all stages of development.

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.

Connexin43 with a cytoplasmic loop deletion inhibits the function of several connexins

Connexins (Cx) form gap junction channels mediating direct intercellular communication. To study the role of amino acids within the cytoplasmic loop, we produced a recombinant adenovirus containing Cx43 with a deletion of amino acids 130-136 (Cx43del(130-136)). Cx43del(130-136) expressed alone in HeLa cells localized within the cytoplasm and did not allow transfer of ions, neurobiotin or Lucifer yellow. When co-expressed with wild type Cx43, Cx43del(130-136) blocked electrical coupling and transfer of neurobiotin or Lucifer yellow. Cx43del(130-136) and Cx43 co-localized by immunofluorescence and were co-purified from Triton X-100-solubilized cell extracts. Intercellular transfer mediated by Cx37 and Cx45 (but not Cx26 or Cx40) was inhibited when co-expressed with Cx43del(130-136). Cx43del(130-136) co-localized with Cx37, Cx40, or Cx45, but not Cx26. These data suggest that Cx43del(130-136) produces connexin-specific inhibition of intercellular communication through formation of heteromeric connexons that are non-functional and/or retained in the cytoplasm.

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.

Functional consequences of heterogeneous gap junction channel formation and its influence in health and disease

The capacity of multiple connexins to hetero-oligomerize into functional heterogeneous gap junction channels has been demonstrated in vivo, in vitro, and in nonmammalian expression systems. These heterogeneous channels display gating activity, channel conductances, selectivity and regulatory behaviors that are sometimes not predicted by the behaviors of the corresponding homogeneous channels. Such observations suggest that heteromerization of gap junction proteins offers an efficient cellular strategy for finely regulating cell-to-cell communication. The available evidence strongly indicates that heterogeneous gap junction assembly is important to normal growth and differentiation, and may influence the appearance of several disease states. Definitive evidence that heterogeneous gap junction channels differentially regulate electrical conduction in excitable cells is absent. This review examines the prevalence, regulation, and implications of gap junction channel hetero-oligomerization.

Connexin43 Associated with an N-cadherin-containing Multiprotein Complex Is Required for Gap Junction Formation in NIH3T3 Cells

Previous studies have indicated an intimate linkage between gap junction and adherens junction formation. It was suggested this could reflect the close membrane-membrane apposition required for junction formation. In NIH3T3 cells, we observed the colocalization of connexin43 (Cx43alpha1) gap junction protein with N-cadherin, p120, and other N-cadherin-associated proteins at regions of cell-cell contact. We also found that Cx43alpha1, N-cadherin, and N-cadherin-associated proteins were coimmunoprecipitated by antibodies to either Cx43alpha1, N-cadherin, or various N-cadherin-associated proteins. These findings suggest that Cx43alpha1 and N-cadherin are coassembled in a multiprotein complex containing various N-cadherin-associated proteins. Studies using siRNA knockdown indicated that cell surface expression of Cx43alpha1 required N-cadherin, and conversely, N-cadherin cell surface expression required Cx43alpha1. Pulse-chase labeling and cell surface biotinylation experiments indicated that in the absence of N-cadherin, Cx43alpha1 cell surface trafficking is blocked. Surprisingly, siRNA knockdown of p120, an N-cadherin-associated protein known to modulate cell surface turnover of N-cadherin, reduced N-cadherin cell surface expression without altering Cx43alpha1 expression. These observations suggest that in contrast to the coregulated cell surface trafficking of Cx43alpha1 and N-cadherin, N-cadherin turnover at the cell surface may be regulated independently of Cx43alpha1. Functional studies showed gap junctional communication is reduced and cell motility inhibited with N-cadherin or Cx43alpha1 knockdown, consistent with the observed loss of both gap junction and cadherin contacts with either knockdown. Overall, these studies indicate that the intracellular coassembly of connexin and cadherin is required for gap junction and adherens junction formation, a process that likely underlies the intimate association between gap junction and adherens junction formation.

Developmental expression patterns and regulation of connexins in the mouse mammary gland: expression of connexin30 in lactogenesis

The mammary gland reaches a fully differentiated phenotype at lactation, a stage characterized by the abundant expression of beta-casein. We have investigated the expression and regulation of gap junction proteins (connexins, Cx) during the various developmental stages of mouse mammary gland. Immunohistochemical analysis, with specific antibodies, reveals that Cx26 and Cx32 are expressed and confined to the cell borders of luminal epithelial cells in all developmental stages of the gland. Cx26 and Cx32 expression, at the mRNA and protein levels, increases in pregnancy and peaks in lactation. Whereas Cx43 mRNA decreases in pregnancy and lactation, the functional activity of Cx43 protein, which has been localized to myoepithelial cells, is regulated (through phosphorylation) during pregnancy and peaks during lactation. Cx30 mRNA and proteins have, for the first time, been detected in mammary gland epithelia. Using reverse transcription/polymerase chain reaction and sequencing techniques, we show that Cx30 is abundant in pregnant and lactating mammary gland. Cx30 protein levels have not been detected in the mammary gland prior to day 15 of pregnancy, whereas maximum expression occurs at the onset of lactation. In mouse mammary cells in culture, Cx30 is epithelial-cell-specific and is induced by lactogenic hormones. These data identify a novel player in mammary differentiation and suggest a potential role for Cx30 in the fully differentiated gland.

CONNEXINS AND CELL SIGNALING IN DEVELOPMENT AND DISEASE

Gap junctions contain hydrophilic membrane channels that allow direct communication between neighboring cells through the diffusion of ions, metabolites, and small cell signaling molecules. They are made up of a hexameric array of polypeptides encoded by the connexin multi-gene family. Cell-cell communication mediated by connexins is crucial to various cellular functions, including the regulation of cell growth, differentiation, and development. Mutations in connexin genes have been linked to a variety of human diseases, including cardiovascular anomalies, peripheral neuropathy, deafness, skin disorders, and cataracts. In addition to their coupling function, recent studies suggest that connexin proteins may also mediate signaling. This could involve interactions with other protein partners that may play a role not only in connexin assembly, trafficking, gating and turnover, but also in the coordinate regulation of cell-cell communication with cell adhesion and cell motility. The integration of these cell functions is likely to be important in the role of gap junctions in development and disease.

Molecular Mechanisms of Epithelial Polarity: About Shapes, Forces, and Orientation Problems

In a variety of organs, epithelial cells assemble into networks of cysts and tubules. Such structures can be reproduced in vitro. Here the importance of plasma membrane compartmentalization and forces that drive morphogenetic events during cystogenesis are discussed.

Analysis of cCx39 expression pattern during chick development

The present study reports the expression pattern of connexin39 (cCx39) in chick embryos at different stages of central nervous system development. We examined the expression between HH17 and HH40 developmental stages of chicken embryos by in situ hybridization (ISH) technique. Connexin39 was first expressed at HH17. It stained neuroepithelial cells in the optic (OV) and telencephalic (TEL) vesicles, plus in the superficial mesenchyme of the two rostral branchial arches (maxilar and mandibular). These cells probably originated from the neural crest. This expression pattern changed drastically between stages HH17 and HH23, while it showed relatively little modifications from HH23 to HH29. At these times, connexin39 was expressed in three regions: the telencephalic vesicle, the diencephalon and the isthmus. At later stages, HH35 and HH40, connexin39 was mainly expressed in the ventricular epithelium and three cell layers of the stratum griseum and fibrosum superficialis (SGFS) in the optic tectum, as well as in granular and nuclear cells in the cerebellum. In conclusion, the expression pattern of connexin39 in embryonic nervous system is dynamic. This pattern is different from, and in some aspects complementary to, those showed by other connexins during brain development.

Short-term effects and teratogenicity of heptanol on embryos of Xenopus laevis

This study examined teratogenic and short-term effects of heptanol on Xenopus embryos. Embryos were exposed for 5h to 2.5mM heptanol at different developmental stages. Teratogenic effects were found in embryos treated from cleavage to early neurula stages. Other heptanol concentrations, shorter exposure time, and the effect of temperature were also assayed. Short-term effects of hepatanol exposure were studied during cleavage and all treated blastulae showed cell separation with delaminated blastomeres inside the blastocel. Disruption of cell adhesion in addition to the uncoupling effect could account for heptanol teratogenicity.

ECM-induced gap junctional communication enhances mammary epithelial cell differentiation

The relationship between gap junctional intercellular communication (GJIC) and mammary cell (CID-9) differentiation in vitro was explored. CID-9 cells differentiate and express beta-casein in an extracellular matrix (ECM)- and hormone-dependent manner. In response to interaction with the ECM, cells in culture modulated the expression of their gap junction proteins at the transcriptional and post-translational levels. In the presence of EHS-matrix, connexins (Cx)26, 32 and 43 localized predominantly to the plasma membrane, and enhanced GJIC [as measured by Lucifer Yellow (LY) dye transfer assays] was noted. Inhibition of GJIC of cells on EHS-matrix with 18 alpha glycyrrhetinic acid (GA) resulted in reversible downregulation of beta-casein expression. In the presence of cAMP, cells cultured on plastic expressed beta-casein, upregulated Cx43 and Cx26 protein levels and enhanced GJIC. This was reversed in the presence of 18 alpha GA. cAMP-treated cells plated either on a non-adhesive PolyHEMA substratum or on plastic supplemented with function-blocking anti-beta 1 integrin antibodies, maintained beta-casein expression. These studies suggest that cell-ECM interaction alone may induce differentiation through changes in cAMP levels and formation of functional gap junctions. That these events are downstream of ECM signalling was underscored by the fact that enhanced GJIC induced partial differentiation in mammary epithelial cells in the absence of an exogenously provided basement membrane and in a beta 1-integrin- and adhesion-independent manner.

The Caenorhabditis elegans innexin INX-3 is localized to gap junctions and is essential for embryonic development

Innexins are the proposed structural components of gap junctions in invertebrates. Antibodies that specifically recognize the Caenorhabditis elegans innexin protein INX-3 were generated and used to examine the patterns of inx-3 gene expression and the subcellular sites of INX-3 localization. INX-3 is first detected in two-cell embryos, concentrated at the intercellular interface, and is expressed ubiquitously throughout the cellular proliferation phase of embryogenesis. During embryonic morphogenesis, INX-3 expression becomes more restricted. Postembryonically, INX-3 is expressed transiently in several cell types, while expression in the posterior pharynx persists throughout development. Through immuno-EM techniques, INX-3 was observed at gap junctions in the adult pharynx, providing supporting evidence that innexins are components of gap junctions. An inx-3 mutant was isolated through a combined genetic and immunocytochemical screen. Homozygous inx-3 mutants exhibit defects during embryonic morphogenesis. At the comma stage of early morphogenesis, variable numbers of cells are lost from the anterior of inx-3(lw68) mutants. A range of terminal defects is seen later in embryogenesis, including localized rupture of the hypodermis, failure of the midbody to elongate properly, abnormal contacts between hypodermal cells, and failure of the pharynx to attach to the anterior of the animal.

Embryonic wound healing by apical contraction and ingression in Xenopus laevis

We have characterized excisional wounds in the animal cap of early embryos of the frog Xenopus laevis and found that these wounds close accompanied by three distinct processes: (1) the assembly of an actin purse-string in the epithelial cells at the wound margin, (2) contraction and ingression of exposed deep cells, and (3) protrusive activity of epithelial cells at the margin. Microsurgical manipulation allowing fine control over the area and depth of the wound combined with videomicroscopy and confocal analysis enabled us to describe the kinematics and challenge the mechanics of the closing wound. Full closure typically occurs only when the deep, mesenchymal cell-layer of the ectoderm is left intact; in contrast, when deep cells are removed along with the superficial, epithelial cell-layer of the ectoderm, wounds do not close. Actin localizes to the superficial epithelial cell-layer at the wound margin immediately after wounding and forms a contiguous "purse-string" in those cells within 15 min. However, manipulation and closure kinematics of shaped wounds and microsurgical cuts made through the purse-string rule out a major force-generating role for the purse-string. Further analysis of the cell behaviors within the wound show that deep, mesenchymal cells contract their apical surfaces and ingress from the exposed surface. High resolution time-lapse sequences of cells at the leading edge of the wound show that these cells undergo protrusive activity only during the final phases of wound closure as the ectoderm reseals. We propose that assembly of the actin purse-string works to organize and maintain the epithelial sheet at the wound margin, that contraction and ingression of deep cells pulls the wound margins together, and that protrusive activity of epithelial cells at the wound margin reseals the ectoderm and re-establishes tissue integrity during wound healing in the Xenopus embryonic ectoderm.

Changing patterns of ganglion cell coupling and connexin expression during chick retinal development

We have used dye injection and immunolabeling to investigate the relationship between connexin (Cx) expression and dye coupling between ganglion cells (GCs) and other cells of the embryonic chick retina between embryonic days 5 and 14 (E5-14). At E5, GCs were usually coupled, via soma-somatic or dendro-somatic contacts, to only one or two other cells. Coupling increased with time until E11 when GCs were often coupled to more than a dozen other cells with somata in the ganglion cell layer (GCL) or inner nuclear layer (INL). These coupled clusters occupied large areas of the retina and coupling was via dendro-dendritic contacts. By E14, after the onset of synaptogenesis and at a time of marked cell death, dye coupling was markedly decreased with GCs coupled to three or four partners. At this time, coupling was usually to cells of the same morphology, whereas earlier coupling was heterogeneous. Between E5 and E11, GCs were sometimes coupled to cells of neuroepithelial morphology that spanned the thickness of the retina. The expression of Cx 26, 32, and 43 differed and their distribution changed during the period studied, showing correlation with events such as proliferation, migration, and synaptogenesis. These results suggest specific roles for gap junctions and Cx's during retinal development.

NBT-II carcinoma behaviour is not dependent on cell-cell communication through gap junctions

To study the mechanism(s) underlying the proliferation of heterogeneous cell populations within a solid tumour, the NBT-II rat bladder carcinoma system was used. It has been first investigated whether the different cell populations are coupled through gap junctions (GJIC). Cells overexpressing the Cx43 were generated to test for any tumour suppressive activity in vivo. To determine whether GJIC is essential for tumour proliferation and the establishment of a cooperative community effect, NBT-II cells that are incompetent for cell coupling were generated. The data report that (i) carcinoma cells expressing or not FGF-1 are coupled through GJIC in vitro and in coculture and express the gap junction protein Cx43, (ii) overexpression of Cx43 in these cells does not affect their in vitro coupling capacities and in vivo tumourigenic growth properties, (iii) inhibition of GJIC through antisense strategy has no in vivo obvious consequence on the tumour growth properties of the carcinoma, and (iv) the community effect between two carcinoma cell populations does not critically involve cell coupling through gap junctions.

Gap junctions and tumour progression

Gap junctional intercellular communication has been implicated in growth control and differentiation. The mechanisms by which connexins, the gap junction proteins, act as tumor suppressors are unclear. In this review, several different mechanisms are considered. Since transformation results in a loss of the differentiated state, one mechanism by which gap junctions may control tumour progression is to promote or enhance differentiation. Processes of differentiation and growth control are mediated at the genetic level. Thus, an alternative or complimentary mechanism of tumour suppression could involve the regulation of gene expression by connexins and gap junctional coupling. Finally, gap junction channels form a conduit between cells for the exchange of ions, second messengers, and small metabolites. It is clear that the sharing of these molecules can be rather selective and may be involved in growth control processes. In this review, examples will be discussed that provide evidence for each of these mechanisms. Taken together, these findings point to a variety of mechanims by which connexins and the gap junction channels that they form may control tumour progression.

Calcium signaling during convergent extension in Xenopus

BACKGROUND

During Xenopus gastrulation, cell intercalation drives convergent extension of dorsal tissues. This process requires the coordination of motility throughout a large population of cells. The signaling mechanisms that regulate these movements in space and time remain poorly understood.

RESULTS

To investigate the potential contribution of calcium signaling to the control of morphogenetic movements, we visualized calcium dynamics during convergent extension using a calcium-sensitive fluorescent dye and a novel confocal microscopy system. We found that dramatic intercellular waves of calcium mobilization occurred in cells undergoing convergent extension in explants of gastrulating Xenopus embryos. These waves arose stochastically with respect to timing and position within the dorsal tissues. Waves propagated quickly and were often accompanied by a wave of contraction within the tissue. Calcium waves were not observed in explants of the ventral marginal zone or prospective epidermis. Pharmacological depletion of intracellular calcium stores abolished the calcium dynamics and also inhibited convergent extension without affecting cell fate. These data indicate that calcium signaling plays a direct role in the coordination of convergent extension cell movements.

CONCLUSIONS

The data presented here indicate that intercellular calcium signaling plays an important role in vertebrate convergent extension. We suggest that calcium waves may represent a widely used mechanism by which large groups of cells can coordinate complex cell movements.

Innexins get into the gap

Connexins were first identified in the 1970s as the molecular components of vertebrate gap junctions. Since then a large literature has accumulated on the cell and molecular biology of this multi-gene family culminating recently in the findings that connexin mutations are implicated in a variety of human diseases. Over two decades, the terms "connexin" and "gap junction" had become almost synonymous. In the last few years a second family of gap-junction genes, the innexins, has emerged. These have been shown to form intercellular channels in genetically tractable invertebrate organisms such as Drosophila melanogaster and Caenorhabditis elegans. The completed genomic sequences for the fly and worm allow identification of the full complement of innexin genes in these two organisms and provide valuable resources for genetic analyses of gap junction function.

Inhibition of endothelial wound repair by dominant negative connexin inhibitors

Wounding of endothelial cells is associated with altered direct intercellular communication. To determine whether gap junctional communication participates to the wound repair process, we have compared connexin (Cx) expression, cell-to-cell coupling and kinetics of wound repair in monolayer cultures of PymT-transformed mouse endothelial cells (clone bEnd.3) and in bEnd.3 cells expressing different dominant negative Cx inhibitors. In parental bEnd.3 cells, mechanical wounding increased expression of Cx43 and decreased expression of Cx37 at the site of injury, whereas Cx40 expression was unaffected. These wound-induced changes in Cx expression were associated with functional changes in cell-to-cell coupling, as assessed with different fluorescent tracers. Stable transfection with cDNAs encoding for the chimeric connexin 3243H7 or the fusion protein Cx43-betaGal resulted in perturbed gap junctional communication between bEnd.3 cells under both basal and wounded conditions. The time required for complete repair of a defined wound within a confluent monolayer was increased by ~50% in cells expressing the dominant negative Cx inhibitors, whereas other cell properties, such as proliferation rate, migration of single cells, cyst formation and extracellular proteolytic activity, were unaltered. These findings demonstrate that proper Cx expression is required for coordinated migration during repair of an endothelial wound.

Roles for alpha 1 connexin in morphogenesis of chick embryos revealed using a novel antisense approach

Gap junctional communication has been implicated in embryonic development and pattern formation. The gap junction protein, alpha 1 connexin (Cx43) is expressed in dynamic and spatially restricted patterns in the developing chick embryo and its expression correlates with many specific developmental events. High levels of expression are found in regions of budding, which leads to shaping and appears to be a necessary prelude for tissue fusions. In order to investigate the role of alpha 1 connexin in these morphogenetic events, we developed a novel method of applying unmodified antisense deoxyoligonucleotides (ODNs) to chick embryos. The use of pluronic gel to deliver antisense ODNs has allowed us to regulate the expression of alpha 1 connexin protein, both spatially and temporally. This "knockdown" results in some striking developmental defects that mimic some common congenital abnormalities, such as spina bifida, anencephaly, myeloschisis, limb malformation, cleft palate, failure of hematopoiesis, and cardiovascular deformity. The results imply a major role for alpha 1 connexin communication in the integration of signaling required for pattern formation during embryonic development. This novel antisense technique may also be widely applicable.

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.

Loss of alpha 1 connexin does not alter the prenatal differentiation of pancreatic beta cells and leads to the identification of another islet cell connexin

Connexin alpha 1, also referred to as Cx43, has thus far been the only gap junction protein identified between the hormone-producing cells of pancreatic islets. To investigate whether loss of this connexin affects the development of endocrine pancreas and the differentiation of insulin-producing beta cells, we have taken advantage of a transgenic line in which the gene coding for connexin alpha 1 had been functionally deleted by homologous recombination. Analysis of pancreas at embryonal day 19.5 (E 19.5) after immunostaining for the four main types of islet hormones, showed that islet cell development was similar in homozygous transgenic mice that completely lacked alpha 1 connexin, in mice that were heterozygous for the transgene, and in age-matched controls with a genetic background similar to that of the transgenic animals. In particular, the three animal groups featured beta cells that had a similar insulin content and ultrastructural organization, including the presence of typical gap junction plaques on the membrane. However, quantitative analysis of freeze-fractured membranes showed that these plaques were less frequent in the transgenic mice lacking alpha 1 connexin. This finding prompted us to revisit the connexin pattern of normal pancreatic beta cells. Using RT-PCR amplification and primers specific for nine of the mammalian connexins, we have found that normal rat and mouse pancreas contain six connexin transcripts, including one that codes for alpha 6 connexin, a protein also referred to as Cx45. This transcript was also identified in isolated pancreatic islets, in FACS-purified suspensions of primary beta cells and in the insulin-producing cells of an experimental tumor. Using antibodies, we found that connexin alpha 6 is expressed by the latter cells, as well as by pancreatic fibroblasts and epithelial duct cells. The data show that pancreatic islets have a normal prenatal development in mice that no longer express alpha 1 connexin. They further provide evidence that normal and tumoral insulin-producing cells natively coexpress connexins alpha 1 and alpha 6.

Developmental expression and assembly of connexins into homomeric and heteromeric gap junction hemichannels in the mouse mammary gland

During the development of the mammary gland, duct-lining epithelial cells progress through a program of expansive proliferation, followed by a terminal differentiation that allows for the biosynthesis and secretion of milk during lactation. The role of gap junction proteins, connexins, in the development and function of this secretory epithelium was investigated. Connexins, Cx26 and Cx32, were differentially expressed throughout pregnancy and lactation in alveolar cells. Cx26 poly-(A)(+) RNA and protein levels increased from early pregnancy, whereas Cx32 was detectable only during lactation. At this time, immunolocalization of connexins by confocal microscopy and immunogold labeling of high-pressure frozen freeze-substituted tissue showed that both connexins colocalized to the same junctional plaque. Analysis of gap junction hemichannels (connexons) isolated from lactating mammary gland plasma membranes by a rate-density centrifugation procedure, followed by immunoprecipitation and by size-exclusion chromatography, showed that Cx26 and Cx32 were organized as homomeric and heteromeric connexons. Structural diversity in the assembly of gap junction hemichannels demonstrated between pregnant and lactating mammary gland may account for differences in ionic and molecular signaling that may physiologically influence the onset and/or maintenance of the secretory phenotype of alveolar epithelial cells.

Connexin channels in Schwann cells and the development of the X-linked form of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease comprises a group of genetically heterogenous disorders of the peripheral nervous system. The X-linked form of Charcot-Marie-Tooth (CMTX) is associated with mutations in the gene encoding the gap junction protein connexin32 (Cx32), which is expressed in Schwann cells. Immunocytochemical evidence suggests that Cx32 is localized to the incisures of Schmidt-Lanterman and the paranodes of myelinating Schwann cells, where it appears to form reflexive gap junctions. It is currently thought that this cytoplasmic continuity provides a much shorter diffusion pathway for the transport of ions, metabolites and second messenger molecules through intracellular channels between the adaxonal and peri-nuclear regions of Schwann cells, across the myelin sheath. This review summarizes our current understanding of the role of connexins in Schwann cells and focuses on the lessons for channel function and disease pathophysiology derived from the functional analysis of Cx32 mutations. One of the most intriguing aspects emerging from this work is that several mutations retain functional competence, although the mutated channels exhibit altered gating properties. This suggests that partial and/or selective disruption of the radial communication pathway formed by Cx32 is sufficient to cause a functional deficit and lead to the development of CMTX. The next challenge will be to define, at the molecular level, the sequence of events involved in the disease process. The presence of a group of functional mutations should help understand the cellular basis of CMTX, by allowing the identification of the specific molecules that need to be exchanged through Cx32 channels, but are excluded from the mutated ones.

Innexin-3 forms connexin-like intercellular channels

Innexins comprise a large family of genes that are believed to encode invertebrate gap junction channel-forming proteins. However, only two Drosophila innexins have been directly tested for the ability to form intercellular channels and only one of those was active. Here we tested the ability of Caenorhabditis elegans family members INX-3 and EAT-5 to form intercellular channels between paired Xenopus oocytes. We show that expression of INX-3 but not EAT-5, induces electrical coupling between the oocyte pairs. In addition, analysis of INX-3 voltage and pH gating reveals a striking degree of conservation in the functional properties of connexin and innnexin channels. These data strongly support the idea that innexin genes encode intercellular channels.

Suppression of human prostate cancer cell growth by forced expression of connexin genes

The cell-to-cell channels in gap junctions, formed of proteins called connexins (Cxs), provide a direct intercellular pathway for the passage of small signaling molecules (< or = 1 kD) between the cytoplasmic interiors of adjoining cells. It has been proposed that alteration in the expression and function of Cxs may be one of the genetic changes involved in the initiation of neoplasia. To elucidate the role of Cxs in the pathogenesis of human prostate cancer (PCA), the pattern of expression of Cx alpha 1 (Cx43) and Cx beta 1 (Cx32) was studied by immunocytochemical analysis in normal prostate and in prostate tumors of different histological grades. While normal prostate epithelial cells expressed only Cx beta 1, both Cx alpha 1 and Cx beta 1 were detected in PCA cells. The Cxs were localized at the cell-cell contact areas in normal prostate and well-differentiated prostate tumors; however, as prostate tumors progressed to more undifferentiated stages, the Cxs were localized in the cytoplasm, followed by an eventual loss in advanced stages. Thus, epithelial cells from prostate tumors showed subtle and gross alterations with regard to expression of Cx alpha 1 and Cx beta 1 and their assembly into gap junctions during the progression of PCA. Retroviral-mediated transfer of Cx alpha 1 and Cx beta 1 into a Cx-deficient human PCA cell line, LNCaP, inhibited growth, retarded tumorigenicity, and induced differentiation, and these effects were contingent upon the formation of gap junctions. In addition, the capacity to form gap junctions in most Cx-transduced LNCaP cells was lost upon serial passage. Taken together, these findings indicate that the control of proliferation and differentiation of epithelial cells in prostate tumors may depend on the appropriate assembly of Cx beta 1 and Cx alpha 1 into gap junctions and that the development of PCA may involve the positive selection of cells with an impaired ability to form gap junctions.

Heart malformations in transgenic mice exhibiting dominant negative inhibition of gap junctional communication in neural crest cells

Transgenic mice were generated expressing an alpha1 connexin/beta-galactosidase fusion protein previously shown to exert dominant negative effects on gap junctional communication. RNase protection analysis and assays for beta-galactosidase enzymatic activity showed that the transgene RNA and protein are expressed in the embryo and adult tissues. In situ hybridization analysis revealed that in the embryo, expression was predominantly restricted to neural crest cells and their progenitors in the dorsal neural tube, regions where the endogenous alpha1 connexin gene is also expressed. Dye-coupling analysis indicated that gap junctional communication was inhibited in the cardiac neural crest cells. All of the transgenic lines were homozygote inviable, dying neonatally and exhibiting heart malformations involving the right ventricular outflow tract-the same region affected in the alpha1 connexin knockout mice. As in the knockout mice, the conotruncal heart malformations were accompanied by outflow tract obstruction. Histological analysis showed that this was associated with abnormalities in the differentiation of the conotruncal myocardium. These results suggest that the precise level of gap junctional communication in cardiac neural crest cells is of critical importance in right ventricular outflow tract morphogenesis. Consistent with this possibility is the fact that cardiac crest cells from the alpha1 connexin knockout mice also exhibited a greatly reduced level of gap junctional communication. These studies show the efficacy of a dominant negative approach for manipulating gap junctional communication in the mouse embryo and demonstrate that targeted expression of this fusion protein can be a powerful tool for examining the role of gap junctions in mammalian development.

Gap junctions in normal and neoplastic mammary gland

Progress in the characterization of gap junctions and their constituent connexin sub-units is leading to a greater understanding of the structure, function, and regulation of this cell-cell communication channel. Although much of the experimental evidence generated to date is correlative, recent work utilizing reverse genetic approaches to manipulate connexin gene function has provided direct evidence that intercellular communication via gap junctions plays key roles in development, cellular differentiation, and organogenesis. Pathogenic mutations in human connexin genes have now been identified. Furthermore, a considerable body of experimental evidence correlates a loss of junctional communication with progression to a malignant phenotype. Although the cell biology of the mammary gland has been extensively studied, the role(s) of gap junctions in the development, differentiation, and maintenance of this tissue are unknown. Gap junctions were first reported in the mammary gland following freeze-fracture and electron microscopic analyses. The development of anti-connexin antibodies and the cloning of individual connexin isoforms have enabled this work to be extended, but there are contradictory reports in the temporal expression patterns of these proteins within mammary epithelium. In addition, a recent report in this Journal has implied by immunocytochemistry that there is up-regulation of connexin protein in some human breast tumours, a novel observation which may be inconsistent with the proposed tumour suppressor role for gap junctions.

Normal differentiation of cultured lens cells after inhibition of gap junction-mediated intercellular communication

The cells of the vertebrate lens are linked to each other by gap junctions, clusters of intercellular channels that mediate the direct transfer of low-molecular-weight substances between the cytosols of adjoining cells. Although gap junctions are detectable in the unspecialized epithelial cells that comprise the anterior face of the organ, both their number and size are greatly increased in the secondary fiber cells that differentiate from them at the lens equator. In other organs, gap junctions have been shown to play an important role in tissue development and differentiation. It has been proposed, although not experimentally tested, that this may be true in the lens as well. To investigate the function of gap junctions in the development of the lens, we have examined the effect of the gap junction blocker 18beta-glycyrrhetinic acid (betaGA) on the differentiation of primary cultures (both dissociated cell-derived monolayers and central epithelium explants) of embryonic chick lens epithelial cells. We found that betaGA greatly reduced gap junction-mediated intercellular transfer of Lucifer yellow and biocytin throughout the 8-day culture period. betaGA did not, however, affect the differentiation of these cells into MP28-expressing secondary fibers. Furthermore, inhibition of gap junctions had no apparent effect on either of the two other types of intercellular (adherens and tight) junctions present in the lens. We conclude that the high level of gap junctional intercellular communication characteristic of the lens equator in vivo is not required for secondary fiber formation as assayed in culture. Up-regulation of gap junctions is therefore likely to be a consequence rather than a cause of lens fiber differentiation and may primarily play a role in lens physiology.

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.

The pattern of disulfide linkages in the extracellular loop regions of connexin 32 suggests a model for the docking interface of gap junctions

Connexins, like true cell adhesion molecules, have extracellular domains that provide strong and specific homophilic, and in some cases, heterophilic interactions between cells. Though the structure of the binding domains of adhesion proteins have been determined, the extracellular domains of connexins, consisting of two loops of approximately 34-37 amino acids each, are not easily studied in isolation from the rest of the molecule. As an alternative, we used a novel application of site-directed mutagenesis in which four of the six conserved cysteines in the extracellular loops of connexin 32 were moved individually and in all possible pairwise and some quadruple combinations. This mapping allowed us to deduce that all disulfides form between the two loops of a single connexin, with the first cysteine in one loop connected to the third of the other. Furthermore, the periodicity of movements that produced functional channels indicated that these loops are likely to form antiparallel beta sheets. A possible model that could explain how these domains from apposed connexins interact to form a complete channel is discussed.

Molecular and functional analysis of cadherin-based adherens junctions

Adherens junctions are specialized forms of cadherin-based adhesive contacts important for tissue organization in developing and adult organisms. Cadherins form protein complexes with cytoplasmic proteins (catenins) that convert the specific, homophilic-binding capacity of the extracellular domain into stable cell adhesion. The extracellular domains of cadherins form parallel dimers that possess intrinsic homophilic-binding activity. Cytoplasmic interactions can influence the function of the ectodomain by a number of potential mechanisms, including redistribution of binding sites into clusters, providing cytoskeletal anchorage, and mediating physiological regulation of cadherin function. Adherens junctions are likely to serve specific, specialized functions beyond the basic adhesive process. These functions include coupling cytoskeletal force generation to strongly adherent sites on the cell surface and the regulation of intracellular signaling events.

Assembly of chimeric connexin-aequorin proteins into functional gap junction channels. Reporting intracellular and plasma membrane calcium environments

Chimeric proteins comprising connexins 26, 32, and 43 and aequorin, a chemiluminescent calcium indicator, were made by fusing the amino terminus of aequorin to the carboxyl terminus of connexins. The retention of function by the chimeric partners was investigated. Connexin 32-aequorin and connexin 43-aequorin retained chemiluminescent activity whereas that of connexin 26-aequorin was negligible. Immunofluorescent staining of COS-7 cells expressing the chimerae showed they were targeted to the plasma membrane. Gap junction intercellular channel formation by the chimerae alone and in combination with wild-type connexins was investigated. Stable HeLa cells expressing connexin 43-aequorin were functional, as demonstrated by Lucifer yellow transfer. Paris of Xenopus oocytes expressing connexin 43-aequorin were electrophysiologically coupled, but those expressing chimeric connexin 26 or 32 showed no detectable levels of coupling. The formation of heteromeric channels constructed of chimeric connexin 32 or connexin 43 and the respective wild-type connexins was inferred from the novel voltage gating properties of the junctional conductance. The results show that the preservation of function by each partner of the chimeric protein is dictated mainly by the nature of the connexin, especially the length of the cytoplasmic carboxyl-terminal domain. The aequorin partner of the connexin 43 chimera reported calcium levels in COS-7 cells in at least two different calcium environments.

Changing patterns of gap junctional intercellular communication and connexin distribution in mouse epidermis and hair follicles during embryonic development

In the mouse embryo between embryonic days 12 (E12) and 16, regular arrays of epidermal placodes on the mystacial pad develop into whisker follicles. This system was chosen for analysis of gap junctional intercellular communication during differentiation. The patterns of communication were studied by microinjection of the tracers Lucifer yellow-CH (LY-CH) and neurobiotin (NB), while immunofluorescent staining was used to study distribution of connexins 26 and 43. Extensive communication was seen between keratinocytes in developing hair pegs or, in later-stage hair follicles, in the germinative matrix. Coupling between adjacent hair pegs via interfollicular epidermis was not observed. Coupling also became restricted as follicular cells differentiated to form outer root sheath, inner root sheath, and hair shaft. Extensive gap junctional coupling is characteristic of keratinocytes that are rapidly proliferating (as in hair pegs and germinative matrix). Follicular keratinocytes commence differentiation shortly before restriction of gap junctional coupling becomes evident. Dermal mesenchymal cells undergoing different modes of differentiation also exhibit differences in gap junctional coupling, as evidenced by poor transfer of LY-CH between cells in dermal condensations of hair follicles compared with extensive transfer elsewhere in the dermis. LY-CH and NB were not transferred between epidermal or follicular epithelium and mesenchyme, arguing against a direct role for gap junctions permeable to known second messenger molecules or nucleotides in epithelial-mesenchymal interactions in this system. The distribution of connexins 26 and 43 in epidermis and hair follicles changed during differentiation but there was no correlation with changing patterns of dye transfer, indicating an unexpected degree of complexity in the relationship between gap junctional intercellular communication and connexin protein distribution during development.

COOH terminus of occludin is required for tight junction barrier function in early Xenopus embryos

Occludin is the only known integral membrane protein localized at the points of membrane- membrane interaction of the tight junction. We have used the Xenopus embryo as an assay system to examine: (a) whether the expression of mutant occludin in embryos will disrupt the barrier function of tight junctions, and (b) whether there are signals within the occludin structure that are required for targeting to the sites of junctional interaction. mRNAs transcribed from a series of COOH-terminally truncated occludin mutants were microinjected into the antero-dorsal blastomere of eight-cell embryos. 8 h after injection, the full-length and the five COOH-terminally truncated proteins were all detected at tight junctions as defined by colocalization with both endogenous occludin and zonula occludens-1 demonstrating that exogenous occludin correctly targeted to the tight junction. Importantly, our data show that tight junctions containing four of the COOH-terminally truncated occludin proteins were leaky; the intercellular spaces between the apical cells were penetrated by sulfosuccinimidyl-6-(biotinamido) Hexanoate (NHS-LC-biotin). In contrast, embryos injected with mRNAs coding for the full-length, the least truncated, or the soluble COOH terminus remained impermeable to the NHS-LC-biotin tracer. The leakage induced by the mutant occludins could be rescued by coinjection with full-length occludin mRNA. Immunoprecipitation analysis of detergent-solubilized embryo membranes revealed that the exogenous occludin was bound to endogenous Xenopus occludin in vivo, indicating that occludin oligomerized during tight junction assembly. Our data demonstrate that the COOH terminus of occludin is required for the correct assembly of tight junction barrier function. We also provide evidence for the first time that occludin forms oligomers during the normal process of tight junction assembly. Our data suggest that mutant occludins target to the tight junction by virtue of their ability to oligomerize with full-length endogenous molecules.

Dominant inhibition of intercellular communication by two chimeric connexins

1. The physiological significance of communication through gap junction channels has been difficult to assess because channel activity cannot be experimentally modulated in a specific manner. To address this problem we have constructed chimeric connexins that function as dominant-negative inhibitors of intercellular channel activity.