Immunocytochemical analysis of the cytoskeleton of the human amniotic epithelium

The amniotic epithelium constitutes a diffusion barrier controlling the passage of solutes and water between the amniotic cavity and maternal circulation. With the present immunocytochemical approach, we have shown that several major components of the cyto-skeleton, i.e., actin, alpha-actinin, spectrin and ezrin, are preferentially associated with the apical and lateral cell surfaces of the human amniotic epithelium. Keratins are distributed throughout the entire cytoplasm, whereas vimentin mainly forms a perinuclear scaffold. These findings indicate a role of the various components of the cytoskeleton in the structural integrity and modulation of cell shape and junctional permeability.

The diversity of connexin genes encoding gap junctional proteins

The multigene family of connexins is larger than previously anticipated. Ten different connexin homologous sequences have been characterized in the mouse genome, five of which are probably the mouse analogues of the known rat connexins26, -31, -32, -43, and -46. Since the additional 5 sequences have been isolated as cDNAs or hybridize specifically to distinct mRNA species, they most likely represent functional connexin genes. Since seven of the genomic connexin sequences have been shown to contain no intron in the coding sequence, this may apply to all mammalian connexin genes. Some of the structural features based on amino acid sequences deduced from cDNA or genomic sequences and the RNA expression pattern of the new connexins are compared with previously described connexins. The structural diversity of the connexin genes suggests that they fulfill different functions coordinated with, and perhaps required for, different programs of cellular differentiation.

Altered patterns of gap junction distribution in ischemic heart disease. An immunohistochemical study of human myocardium using laser scanning confocal microscopy

Arrhythmias are a common and potentially life-threatening complication of myocardial ischemia and infarction in humans. The structural pathways for the rapid intercellular conduction of the electrical impulse that stimulates coordinated contraction in the myocardium are formed by the gap junctions situated at intercalated disks. By raising antibodies to cardiac gap-junctional protein, and using these antibodies in an immunohistochemical procedure in combination with the technique of laser scanning confocal microscopy, we have succeeded in localizing gap junctions, with a clarity not previously possible, through thick volumes of human myocardial tissue. To explore the structural basis for ischemia and infarction-related arrhythmogenesis, antibody labeling and laser scanning confocal microscopy were applied to study the organization, distribution, and other characteristics of gap junctions in the explanted hearts of patients undergoing cardiac transplantation for advanced ischemic heart disease. In areas of myocardium free from histologically detectable structural damage, there was no significant difference in the size of distribution of labeled gap junctions, or in their number per intercalated disk, between left ventricular tissue (in which functional impairment was severe) and right ventricular tissue (in which functional impairment was minimal). However, in myocytes at the border of healed infarcts--zones to which the slow conduction responsible for reentry arrhythmias has been localized--the organization of gap junctions was markedly disordered; instead of being aggregated into discrete intercalated disks, gap-junctional immunostaining was spread extensively over myocyte surfaces. Some infarct zones were bridged by continuous strands of myocytes, coupled to one another by gap junctions, thereby linking healthy myocardium on either side. At their thinnest, these bridges were in some instances no wider than a single attenuated myocyte. The conclusions are 1) a widespread, generalized derangement of gap junction organization does not appear to underlie functional impairment in the ischemic heart, 2) a disorderly arrangement typifies gap junctions in myocytes of the infarct border zone, and this may contribute to alterations in conduction that are capable of precipitating reentry arrhythmias, and 3) delicate chains of myocytes traverse some healed infarcts, apparently forming electrically coupled bridges across what would otherwise constitute blocked zones. The weakest link in this chain can be a single, degenerating myocyte; avoidance of arrhythmia may therefore depend on the continued survival of this single cell.

Evidence that vinculin is co-distributed with actin bundles in ectoplasmic ("junctional") specializations of mammalian Sertoli cells

Ectoplasmic specializations of Sertoli cells are actin containing structures found at sites of attachment to spermatids and to neighboring Sertoli cells. We suspect that these cytoskeletal structures are a form of actin-associated adhesion junction. If this is true, then molecular components, such as vinculin, that characterize actin-associated adhesion junctions in general should be present in ectoplasmic specializations. In this paper we have used two approaches to verify the prediction that vinculin is a component of ectoplasmic specializations. First, we have used fluorescence microscopy to probe immunologically for vinculin in ectoplasmic specializations associated with spermatids of the ground squirrel. Second, we have used immunogold techniques to probe for vinculin in ectoplasmic specializations of rat testis. Our results indicate that the immunological probe for vinculin was reactive with ectoplasmic specializations. In single label fluorescence experiments, linear patterns obtained with the vinculin probe were similar to those obtained with probes for filamentous actin. In double label experiments, the vinculin probe was co-distributed with the actin probes. In immunogold studies, specific labelling with the probe for vinculin occurred in ectoplasmic specializations both at sites of attachment to spermatids and adjacent to basal Sertoli cell junctions. Moreover, gold particles were concentrated adjacent to filament bundles within each ectoplasmic specialization. Our results support the conclusion that vinculin is present in ectoplasmic specializations. Further, they indicate that vinculin is co-distributed with actin bundles within each ectoplasmic specialization.

The tight-junction-specific protein ZO-1 is a component of the human and rat blood-brain barriers

Continuous tight junctions between vascular endothelial cells, the principal anatomical basis for the blood-brain barrier, have been investigated functionally and morphologically but their molecular components have not been defined. This communication reports that the protein ZO-1, a specific constituent of epithelial tight junctions, is found in human and rat brain vasculature. ZO-1-positive immunocytochemical staining forms a tightly banded pattern outlining individual endothelial cells in blood vessels of the human cerebral cortex. Rat brain exhibits a similar staining of blood vessels as well as ZO-1-positive staining around individual epithelial cells of the choroid plexus. The antiserum used for immunocytochemistry recognizes a protein of about 200 kDa in rat brain microvessels by Western blot. These findings indicate that ZO-1 is located at the interendothelial junctions of brain vasculature, implicating its importance as a component of the blood-brain barrier.

Immunoelectron microscopic localization of cell surface antigens on rat hepatocytes detected with monoclonal antibodies (HAM2 and HAM4)

The localization of surface antigens and the binding activity of two monoclonal antibodies, HAM2 and HAM4, which recognize the rat major histocompatibility complex (MHC) antigen class I and the rat hepato-renal antigen respectively, on dissociated (free) hepatocytes was examined by light (LM) and electron microscopy (EM), and by radioimmunoassay (RIA). Fixed hepatocytes, fixed before dissociation, and fresh hepatocytes, dissociated by collagenase, were treated by direct staining with HAM2- or HAM4-immunogold complexes (HAM2-gold and HAM4-gold). Some of the directly stained hepatocytes were further mixed with antimouse IgG-gold complex (IgG-gold) to supplement the direct staining. The polarity of the sinusoidal and contiguous faces and the bile canaliculus, i.e. the in situ morphology, was well preserved in the fixed hepatocytes, while the fresh cells had lost the polarity and were round. On the fixed hepatocytes HAM2-gold particles were distributed predominantly on the sinusoidal face, while HAM4-gold particles were localized on both the bile canalicular and sinusoidal faces. No different antigen distribution on the fresh cells was detected with the two antibodies. Supplementation by IgG-gold was noticeable in most cases. The extent of binding activity in both the immunogold and RIA experiments was lower in the fixed cells than in fresh cells. These results suggest that HAM2 and HAM4 are useful monoclonal antibodies for detecting the localization of the MHC class I antigen and the hepato-renal antigen on the hepatocytes, respectively.

Evidence for the presence of actin-associated intercellular adhesion junction between interstitial cells of Leydig in the ground squirrel testis

Interstitial cells of Leydig characteristically occur in clusters around blood vessels. Often these clusters remain intact when interstitial tissues are mechanically separated from other components of the testis. The presence of strong intercellular attachments is most likely one of the factors responsible for maintaining the integrity of Leydig cell clusters. In many tissues, actin associated adhesion junctions commonly provide intercellular attachment. To determine if actin associated adhesion junctions are present between Leydig cells, we have used 1) immunofluorescence to probe for two components that characterize these junctions in other tissues and 2) electron microscopy to examine areas of intercellular contact for evidence of microfilament related adhesion junctions. Isolated clusters of unsectioned cells, which had been fixed and detergent extracted, were probed with the F-actin specific strains rhodamine phalloidin and NBD-phallacidin and with an affinity purified primary antibody raised against human platelet vinculin. In regions of intercellular contact, fluorescence staining with the actin probes was intense and appeared as a solid linear band. Similar regions also stained with the vinculin probe. In double label experiments, actin and vinculin probes were co-distributed at sites of intercellular contact. Zones of intercellular contact, apparently similar to those detected with fluorescence microscopy, were observed at the ultrastructural level. At these sites, subsurface filaments, interpreted by us as actin, formed a dense carpet adjacent to the plasma membrane on each side of the junction. These filaments appeared to be organized into networks rather than discrete bundles.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Polarized expression of integrin receptors (alpha 6 beta 4, alpha 2 beta 1, alpha 3 beta 1, and alpha v beta 5) and their relationship with the cytoskeleton and basement membrane matrix in cultured human keratinocytes

In human keratinocytes cultured in conditions which allow differentiation and stratification and are suitable to reconstitute a fully functional epidermis, alpha 6 beta 4 and two members of the beta 1 integrin family (alpha 2 beta 1 and alpha 3 beta 1) were respectively polarized to the basal and lateral domains of the plasmamembrane both in growing colonies and in the reconstituted epidermis. Conversely, the alpha v integrin subunit, presumably in association with beta 5, was expressed at the basal surface in growing and migrating but not in stationary keratinocytes. The integrin alpha 6 beta 4: (a) was organized in typical patches which often showed a "leopard skin" pattern where spots corresponded to microfilament-free areas; (b) was not associated with focal contacts containing vinculin and talin but rather corresponded to relatively removed contact areas of the basal membrane as shown by interference reflection microscopy; and (c) was coherent to patches of laminin secreted and deposited underneath the ventral membrane of individual cells. The two beta 1 integrins (alpha 2 beta 1 and alpha 3 beta 1), both endowed with laminin receptor properties, were not associated with focal adhesions under experimental conditions allowing full epidermal maturation but matched the lateral position of vinculin (but not talin), cingulin, and desmoplakin, all makers of intercellular junctions. Often thin strips of laminin were observed in between the lateral aspects of individual basal keratinocytes. The integrin complex alpha v beta 5 had a topography similar to that of talin- and vinculin-containing focal adhesions mostly in the peripheral cells of expanding keratinocyte colonies and in coincidence with fibronectin strands. The discrete topography of beta 1 and beta 4 integrins has a functional role in the maintenance of the state of aggregation of cultured keratinocytes since lateral aggregation was impaired by antibodies to beta 1 whereas antibodies to beta 4 prevented cell-matrix adhesion (De Luca, M., R. N. Tamura, S. Kajiji, S. Bondanza, P. Rossino, R. Cancedda, P. C. Marchisio, and V. Quaranta. Proc. Natl. Acad. Sci. USA. 87:6888-6892). Moreover, the surface polarization of integrins followed attachment and depended both on the presence of Ca2+ in the medium and on the integrity of the cytoskeleton. We conclude that our in vitro functional tests and structural data suggest a correlation between the pattern of integrin expression on defined plasmamembrane domains and the mechanism of epidermal assembly.

Expression of a novel cadherin (EP-cadherin) in unfertilized eggs and early Xenopus embryos

Two distinct cadherin cDNA clones of Xenopus laevis were isolated from a stage 17 embryo cDNA library. Analysis of the complete deduced amino acid sequences indicated that one of these molecules is closely homologous to chicken and mouse N-cadherin, while the other displays comparable homology to both E- and P-cadherins and was thus denoted EP-cadherin. This molecule has an apparent relative molecular mass of 125 x 10(3) (compared to approx. 138 x 10(3) or approx. 140 x 10(3) of E-cadherin and N-cadherins, respectively). Northern and Western blot analyses indicated that N-cadherin is first expressed at the neurula stage while EP-cadherin is the only cadherin detected in unfertilized eggs and cleavage stage embryos. Immunolabeling of Xenopus eggs with antibodies prepared against a fusion protein, containing a segment of EP-cadherin, indicated that the protein is highly enriched at the periphery of the animal hemisphere. EP-cadherin was also found in A6 epithelial cells derived from Xenopus kidneys, and was apparently localized in the intercellular adherens junctions.

Lipid composition of lens plasma membrane fractions enriched in fiber junctions

Little is known about the lipid environment of lens fiber junctions, the plasma membrane structure proposed to be responsible for passage of low molecular weight metabolites between adjacent lens fiber cells. Plasma membranes of the ocular lens are especially rich in fiber junctions. The resistance of junctional domains to disruption by detergent or alkali treatment provides the opportunity to isolate a lens plasma membrane fraction enriched in fiber junctions. When examined by electron microscopy, the fiber junction fraction prepared from bovine lenses was enriched with junctional structures by about twofold when compared to total plasma membrane. We compared the protein, phospholipid, and cholesterol concentration of total plasma membrane with fiber junctional membrane from rat and cow lens and from aged normal cataractous human lenses. The principal finding was that junctional membrane contained 20-40% more total lipid than that of the total plasma membrane. This was due to a proportionate increase in the relative content (mg/mg protein) of both phospholipid and cholesterol. Exclusive of one exception (nucleus of bovine lens), the cholesterol/phospholipid molar ratios of the two fractions were similar. In the bovine nucleus, the cholesterol/phospholipid molar ratio was substantially higher in the fiber junctional-enriched membrane fraction than in the total plasma membrane, suggesting a special association of cholesterol with bovine nuclear fiber junctions. The relative lipid compositions of the plasma membrane and fiber junction-enriched fractions from human normal and cataractous lenses were similar, suggesting that human senile cataractogenesis involves changes in the lens plasma membrane more subtle than would be reflected by gross changes in the membrane lipid composition.

Formation of hemidesmosomes in vitro by a transformed rat bladder cell line

Two hemidesmosomal plaque components of 230 and 180 kD have recently been characterized using autoantibodies in the serum samples of bullous pemphigoid (BP) patients (Klatte, D. H., M. A. Kurpakus, K. A. Grelling, and J. C. R. Jones. 1989, J. Cell Biol. 109:3377-3390). These BP autoantibodies generate the type of staining patterns that one would predict for formed hemidesmosomes, i.e., a punctate staining pattern towards the substratum; in less than 50% of various primary epithelial and transformed epidermal cell lines even when such cells are maintained in culture for prolonged periods. In contrast, affinity-purified human autoantibodies against the 230-kD hemidesmosomal plaque component generate intense immunofluorescence staining along the region of cell-substratum interaction in the rat bladder tumor cell line 804G maintained on uncoated glass cover-slips. This pattern is distinct from that observed in the 804G cells using an antibody preparation directed against vinculin, a component of adhesion plaques. Ultrastructural analyses of the 804G cells reveals that hemidesmosome-like structures occur along the basal surface of cells where they abut the substratum. These structures are present in 804G cells maintained in culture in reduced levels of Ca2+ and are recognized by autoantibodies directed against the 230-kD hemidesmosomal plaque component as determined by immunogold ultrastructural localization. To study hemidesmosome appearance in this cell line, 804G cells were trypsinized and then allowed to readhere to glass coverslips. In rounded, unattached 804G cells, hemidesmosome-like plaque structures occur along the cell surface. These structures are recognized by the 230-kD autoantibodies. At 1 h after plating, hemidesmosomes are observed along the substratum attached surface of cells. Protein synthesis is not required for the appearance of these hemidesmosomes. Within 4 h of plating, autoantibody staining and hemidesmosomes appear towards the cell periphery. Subsequently, the polypeptide recognized by the BP autoantibodies becomes concentrated in the perinuclear region, where there are numerous hemidesmosomes. We propose that the hemidesmosomes in 804G cells are involved in cell-substratum adhesion. We discuss possible mechanisms of assembly of hemidesmosomes in the 804G cells. Indeed, the 804G cells should prove an invaluable cell line for the biochemical and molecular dissection of hemidesmosome structure, function, and assembly.

Reorganization of cytoskeletal and junctional proteins during cochlear hair cell degeneration

Experiments were carried out to elucidate changes in cytoskeletal elements and intercellular junctions in the organ of Corti, when hair cells degenerate and phalangeal scars form. Hair cell damage was induced by exposing guinea pigs to high intensity noise. The spatial and temporal changes in the organization of microfilaments, intermediate filaments, and tight junction-specific proteins were investigated using scanning and transmission electron microscopy and histochemistry. The results show that microfilaments, cytokeratins, adherens junctions, and tight junctions rearrange their distribution in damaged areas. From the temporal sequence of these changes it appears that phalangeal scars develop simultaneous with hair cell degeneration, and that the integrity of the luminal membranes in the organ of Corti is not interrupted. Each scar is formed by two supporting cells which expand and invade the sub-apical region of the dying hair cell. This region becomes cytokeratin-positive. The two supporting cells meet at the mid-line of the scar, where a new junctional complex is formed. The junctional complex consists of tight junction and adherens-type junction, but desmosomes are absent.

Gap junction protein tissue distribution and abundance in the adult brain in Drosophila

The distribution of gap junction (GJ) protein in Drosophila tissues and developmental stages was determined by probing immuno-blots with an anti-Drosophila GJ protein antibody (R2AP18). All tissues and developmental stages examined contained 18, 24 or 72 kD GJ protein. GJ protein was notably abundant in immuno-blots of homogenates of adult brain tissue. This was confirmed by the direct visualization of GJs in thin sections of adult brain by electron microscopy. GJs were particularly large and numerous between glial cells in the optic lobes and peripheral glial sheath. R2AP18 reactivity was used to identify GJ protein in immunoblots of cell fractions from isolated adult heads. The final GJ-enriched pellets, derived by extracting crude membrane fractions with urea and N-lauroyl sarcosine, contained GJs with reduced profile widths (13-15 nm vs 16-18 nm for native GJs) and which, unlike native GJs in the crude membrane fractions, were immuno-labelled by R2AP18. Immuno-blots of the urea-sarcosine extracted GJ pellets and supernatant contained higher molecular weight R2AP18 immuno-reactive proteins in addition to the 18 kD form which was present in the tissue homogenate and crude membrane fractions. The results confirm previous observations that urea-sarcosine causes alterations in GJ structure and suggest that urea-sarcosine treatment exposes antigenic determinant(s) which are unavailable for R2AP18 binding in non-extracted native GJs. The abundance of GJs in the adult brain and the relatively simple R2AP18 staining patterns in immuno-blots of GJ-enriched fractions from isolated adult heads suggest that this tissue will be useful for further biochemical and molecular studies of GJs in Drosophila.

Immunocytochemical localization of the erythroid glucose transporter: abundance in tissues with barrier functions

We investigated the cellular localization and tissue distribution of the glucose transporter protein in the nervous system of the monkey and rat, and in other tissues of the rat, by immunocytochemical methods with monoclonal and polyclonal antibodies to the glucose transporter of human erythrocytes. We found intense immunostaining, indicating a high density of the glucose transporter, in all intraparenchymal blood vessels of the brain and spinal cord, in pial vessels, and in endoneurial capillaries of peripheral nerves, nerve roots, and dorsal root ganglia. Larger blood vessels at the base of the brain and in major fissures did not stain. The only intraparenchymal brain microvessels that did not immunostain were in circumventricular organs. There was no specific immunostaining of neurons or glia, except for tanycytes in the floor of the third ventricle, which immunostained intensely. Vessels of the choroid plexus did not stain, but the choroid epithelium, especially its basal membranes, stained. The only non-neural organ where immunostaining was evident in its microvessels was the testis. In addition to the endothelium of neural and testicular tissues, there was immunostaining in certain epithelial tissues, such as the perineurium of peripheral nerves and nerve roots, the epithelium of the ascending loop of Henle in the kidney, and the epidermis of the skin. Based on these findings, we hypothesize that a high density of the erythroid-type glucose transporter is inherent to many endothelial and epithelial cells that are joined by occluding junctions. However, other epithelial tissues with known occluding intercellular junctions that lack the erythroid-type of glucose transporter may have other types of glucose transporter proteins.

Distinct functions for integrins alpha 3 beta 1 in focal adhesions and alpha 6 beta 4/bullous pemphigoid antigen in a new stable anchoring contact (SAC) of keratinocytes: relation to hemidesmosomes

Basal cells of stratified epidermis are anchored to the basement membrane zone (BMZ) of skin via hemidesmosomes. We previously identified integrin alpha 3 beta 1, in focal adhesions (FAs), of cultured human keratinocytes (HFKs) as a mediator of HFK adhesion to secreted BMZ-like extracellular matrix (ECM; Carter, W.G., E.A. Wayner, T.S. Bouchard, and P. Kaur. 1990. J. Cell Biol. 110: 1387-1404). Here, we have examined the relation of integrins alpha 6 beta 4 and alpha 3 beta 1, to bullous pemphigoid antigen (BPA), a component of hemidesmosomes. We conclude that alpha 6 beta 4 in HFKs localizes in a new stable anchoring contact (SAC) that cooperates with alpha 3 beta 1-FAs to mediate adhesion to ECM, based on the following. (a) Comparison of secreted ECM, with exogenous laminin, fibronectin and collagen identified ECM as the preferred ligand for HFK adhesion and spreading and for formation of both alpha 6 beta 4-SACs and alpha 3 beta 1-FAs. (b) Inhibition of HFK adhesion with combined anti-alpha 3 beta 1 (P1B5) and anti-alpha 6 beta 4 (GoH3) antibodies indicated that both receptors were functional in adhesion to ECM while alpha 3 beta 1 played a dominant role in spreading. (c) alpha 6 beta 4 colocalized with BPA in SACs that were proximal to but excluded from FAs. Both alpha 6 beta 4-SACs and alpha 3 beta 1-FAs were in contact with the adhesion surface as indicated by antibody exclusion and interference reflection microscopy. (d) In contrast to alpha 3 beta 1-FAs, alpha 6 beta 4-SACs were present only in nonmotile cells, not associated with stress fibers, and were relatively stable to detergents and urea, suggesting a nonmotile, or anchoring function for SACs and motility functions for alpha 3 beta 1-FAs. (e) alpha 6 beta 4 formed a detergent-insoluble complex with exogenous ECM in an affinity isolation procedure, confirming the ability of an unidentified ECM ligand to interact with alpha 6 beta 4. (f) We suggest that alpha 6 beta 4/BPA-SACs in culture restrict migration of HFKs on ECM while alpha 3 beta 1-FAs form dynamic adhesions in spreading and migrating cells. alpha 6 beta 4/BPA-SACs in culture bear functional and compositional similarities to hemidesmosomes in skin.

Filipin-sterol complexes in molluscan gill ciliated epithelial cell membranes: intercalation into ciliary necklaces and induction of gap junctional particle arrays

Freeze-fracture electron microscopy has been used in conjunction with the antibiotic filipin to investigate possible differences in the distribution of sterols in ciliary and somatic cell membranes of scallop and mussel gill epithelial cells. Contrary to previous reports, we find that filipin-sterol lesions can occur among the strands of the ciliary necklace but they are partially excluded from the smooth neck region above the necklace where the membrane is tightly apposed to the axonemal microtubules. No obvious differences in filipin-sterol lesions occur in the membranes of mussel gill cilia of varying mechanical sensitivity. Although abundant in the apical plasma membrane, filipin-sterol complexes are rare within the membranes of microvilli. Filipin-sterol lesions form outside the loosely parallel particle strands of septate junctions, sometimes increasing their relative orderliness. At sufficiently high density, filipin-sterol protrusions within the plasma membrane result in mass aggregation of gap junctions, possibly through recruitment of unorganized connexons.

Differential phosphorylation of the gap junction protein connexin43 in junctional communication-competent and -deficient cell lines

Connexin43 is a member of the highly homologous connexin family of gap junction proteins. We have studied how connexin monomers are assembled into functional gap junction plaques by examining the biosynthesis of connexin43 in cell types that differ greatly in their ability to form functional gap junctions. Using a combination of metabolic radiolabeling and immunoprecipitation, we have shown that connexin43 is synthesized in gap junctional communication-competent cells as a 42-kD protein that is efficiently converted to a approximately 46-kD species (connexin43-P2) by the posttranslational addition of phosphate. Surprisingly, certain cell lines severely deficient in gap junctional communication and known cell-cell adhesion molecules (S180 and L929 cells) also expressed 42-kD connexin43. Connexin43 in these communication-deficient cell lines was not, however, phosphorylated to the P2 form. Conversion of S180 cells to a communication-competent phenotype by transfection with a cDNA encoding the cell-cell adhesion molecule L-CAM induced phosphorylation of connexin43 to the P2 form; conversely, blocking junctional communication in ordinarily communication-competent cells inhibited connexin43-P2 formation. Immunohistochemical localization studies indicated that only communication-competent cells accumulated connexin43 in visible gap junction plaques. Together, these results establish a strong correlation between the ability of cells to process connexin43 to the P2 form and to produce functional gap junctions. Connexin43 phosphorylation may therefore play a functional role in gap junction assembly and/or activity.

Changes in the expression of connexin 43, a cardiac gap junctional protein, during mouse heart development

A cDNA probe coding for rat connexin 43 (Beyer et al., 1987), a gap junctional protein, was used to detect specific mRNA and estimate its relative abundance in mouse heart at different developmental stages: 11, 14 and 19 days post-coïtum (dpc); 1, 2 and 3 weeks post-partum (wpp), and at the adult stage. On Northern blots of total cellular RNA, a single 3.0 kb message was detected at all stages of development, and the differential intensities of labeling indicated developmental changes in mRNA abundance. mRNA levels were further investigated by dot-blotting. Densitometric analyses of dot-blot autoradiograms showed a five-fold increase of the mRNA level between 11 dpc and 1 wpp, then a gradual decrease until the adult stage where it reached a value close to that detected at 11 dpc. By comparison, myosin heavy chains and glycerldehyde-3-phosphate dehydrogenase mRNAs were found to peak at 3 wpp and 14 dpc, respectively. The presence and the relative abundance of connexin 43 were investigated at the same developmental stages as previously by immunoblotting of whole-ventricle fractions using antipeptide antibodies specific for this junctional protein. Quantitative data obtained from densitometric analyses of immunoblots showed that from 14 dpc to 1 wpp intensity of labeling of connexin 43 was roughly multiplied by a factor of 10. It peaked at 3 wpp before dropping to about 20% at the adult stage. The data obtained with both the cDNA probe and the antibodies were significant as shown by variance analyses. They suggest that expression of cardiac connexin 43 is developmentally-regulated: at the early stages of heart development the expression levels of the protein would seem to be mainly regulated by mRNA abundance; beyond 2 weeks after birth, the levels of connexin 43 would seem rather to depend upon its stability and/or the efficiency of the translation.

Localization of adherens junction proteins along the possible sliding interface between secretory ameloblasts of the rat incisor

Localization of junctions between inner enamel-secretory ameloblasts was examined by immunofluorescence microscopy using antibodies against adherens junction proteins, radixin, vinculin, and A-CAM. All antibodies used stained the boundary between the ameloblasts exclusively in the plane where F-actin was abundant. This suggests that the adherens junctions in the ameloblasts are involved in cell-to-cell movement with actin-based microfilament bundles.

Characterization of the ZO-1 protein in endothelial and other cell lines

A high molecular weight tight junction-associated protein, ZO-1, has been demonstrated in liver (hepatocytes) and in both epithelium and endothelium. We carried out studies to examine the presence of the protein in vascular endothelial cell cultures and several other types of cultured cells, and the relationship between the ZO-1 protein content and confluency of endothelial cell monolayers. Immunofluorescence labelling of endothelial monolayers and two types of epithelial monolayers, IEC-6 and MDCK, with monoclonal antibody against ZO-1 protein localized the protein to the cell peripheries. Its association with the cell periphery only occurred when cells had contact with one another as demonstrated in endothelial cells. We have been able to show a positive correlation between the ZO-1 content of the cells and the extent of monolayer confluency in the endothelial cells by immunoblotting. The protein is much less expressed in nonconfluent endothelial cell monolayers and totally absent from mouse myeloma cultures. The presence and confluence-related expression of the protein in endothelium give support to the hypothesis that tight junctions exist in confluent endothelial cells and that the ZO-1 protein is expressed under the conditions where tight junction interactions occur.

The gap junction-like form of a vacuolar proton channel component appears not to be an artifact of isolation: an immunocytochemical localization study

Gap junctional structures containing a 16-kDa intrinsic membrane protein have been isolated from the hepatopancreas of the crustacean Nephrops norvegicus. These structures are double membranes 14-15 nm thick and composed of hexagonal arrays of particles which have a central pore that is penetrated by a cationic negative stain. Membrane preparations have also been isolated from the hepatopancreas and these contain similar gap junctional regions of uniform width. Affinity purified antibodies to the 16-kDa protein bind principally to these gap junctional regions. Antiserum raised against the isolated gap junctional structures binds strongly to the lateral surfaces of the columnar epithelial cells and in particular to gap junction-like regions.

Biochemical and immunochemical analysis of the arrangement of connexin43 in rat heart gap junction membranes

A 43 × 10(3) Mr protein (designated connexin43 or Cx43) is a major constituent of heart gap junctions. The understanding of its arrangement in junctional membranes has been extended by means of site-directed antibodies raised against synthetic peptides of Cx43. These represent part of the first extracellular loop (EL-46), the cytoplasmic loop (CL-100), the second extracellular loop (EL-186) and carboxy-terminal sequences (CT-237 and CT-360). All of the antibodies raised reacted with their respective peptides and the Cx43 protein on Western blots. By immunoelectron microscopy two of the antibodies (CL-100 and CT-360) were shown to label the cytoplasmic surface of isolated gap junction membranes. Immunofluorescent labeling at locations of neonatal cardiac myocyte-myocyte apposition required an alkali/urea treatment when the EL-46 and EL-186 antibodies were used. Immunoblot analysis of endoproteinase Lys-C-digested gap junctions revealed that the Cx43 protein passed through the lipid bilayer four times. Alkaline phosphatase digestion of isolated junctions was used to show that the CT-360 antibody recognized many phosphorylated forms of Cx43. Our results unequivocally confirm models of the organization of Cx43 that were based on a more limited set of data and a priori considerations of the sequence.