Preparation of a gap junction fraction from uteri of pregnant rats: the 28-kD polypeptides of uterus, liver, and heart gap junctions are homologous

Molecular structure of the gap junctional channel

The proteins in various gap junctional preparations from rodent liver have been analysed by two-dimensional peptide mapping and immunoblotting. Only the protein of relative molecular mass (Mr) 16,000 (16K) is found in all gap junctional isolates, and it is unrelated to the 27K protein. The absence of the 27K protein and any of its fragments from trypsin-treated preparations suggests that this protein does not directly contribute to gap junctional structure. Peptide mapping and immunoblotting of the 16K proteins isolated from various tissues and species and of the arthropod 18K protein present in gap junctional preparations from Nephrops norvegicus show that these proteins constitute a family of related junctional proteins. A site-specific antiserum raised against the N-terminal octapeptide of the 16K protein from mouse liver cross-reacts with all 16K and 18K forms of the junctional protein so far tested, suggesting that this particular antigenic determinant is highly conserved. Immuno-localization studies show that the N-terminus is most likely located on the cytoplasmic aspect of the junction and is available to Pronase digestion.

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

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

Electron cryo-crystallography of a recombinant cardiac gap junction channel

Gap junctions in the heart play an important functional role by electrically coupling cells, thereby organizing the pattern of current flow to allow co-ordinated muscle contraction. Cardiac gap junctions are therefore intimately involved in normal conduction as well as the genesis of potentially lethal arrhythmias. We recently utilized electron cryo-microscopy and image analysis to examine frozen-hydrated 2D crystals of a recombinant, C-terminal truncated form of connexin 43 (Cx43; alpha 1), the principal cardiac gap junction protein. The projection map at 7 A resolution revealed that each 30 kDa connexin subunit has a transmembrane alpha-helix that lines the aqueous pore and a second alpha-helix in close contact with the membrane lipids. The distribution of densities allowed us to propose a model in which the two apposing connexons that form the channel are staggered by approximately 30 degrees. We are now recording images of tilted, frozen-hydrated 2D crystals, and a preliminary 3D map has been computed at an in-plane resolution of approximately 7.5 A and a vertical resolution of approximately 25 A. As predicted by our model, the two apposing connexons that form the channel are staggered with respect to each other for certain connexin molecular boundaries within the hexamer. Within the membrane interior each connexin subunit displays four rods of density, which are consistent with an alpha-helical conformation for the four transmembrane domains. Preliminary studies of BHK hamster cells that express the truncated Cx43 designated alpha 1 Cx263T demonstrate that oleamide, a sleep inducing lipid, blocks in vivo dye transfer, suggesting that oleamide causes closure of alpha 1 Cx263T channels. The comparison of the 3D structures in the presence and absence of oleamide may provide an opportunity to explore the conformational changes that are associated with oleamide-induced blockage of dye transfer. The structural details revealed by our analysis will be essential for delineating the molecular basis for intercellular current flow in the heart, as well as the general molecular design and functional properties of this important class of channel proteins.

Structure of cardiac gap junction intercellular channels

Gap junction proteins, termed connexins, constitute a multigene family of polytopic membrane channel proteins that have four hydrophobic transmembrane domains with the N- and C-termini located on the cytoplasmic membrane face. The principal gap junction protein in the heart, alpha 1 connexin (also designated Cx43), mediates action potential propagation between cells in order to synchronize cardiac contraction. alpha 1 connexin channels are concentrated in gap junction plaques located in the intercalated disks. The intercellular channel is formed by the docking of two hemi-channels, termed connexons, formed by a ring of six 43-kDa alpha 1 connexin subunits. Each subunit is asymmetric with an axial ratio of 4-5:1 with approximately 20 A extending into the extracellular gap approximately 50 A spanning the lipid bilayer and approximately 50 A extending into the cytoplasmic space. We have recently grown two-dimensional crystals of a recombinant C-terminal truncation mutant of alpha 1 connexin (designated alpha 1Cx263T) that are ordered to better than 7 A resolution. Projection density maps derived by electron cryocrystallography revealed that the intercellular channel is lined by six alpha-helices, and there is a second ring of six alpha-helices at the interface with the membrane lipids. These rings of alpha-helices are staggered by 30 degrees, which predicts that the two connexons in the channel are staggered by 30 degrees such that each connexin subunit in one connexon interacts with two subunits in the apposed connexon. Such a quaternary arrangement may confer stability in the docking of the connexons to form a tight electrical seal for intercellular current flow during cardiac conduction.

Gap junctions in development--a perspective

The status of the gap junction as a pathway for cellular interactions during development is reviewed. Current evidence suggests that gap junctions play an important part in ensuring normal development, although the precise role of gap junctional communication remains to be defined. Communication through gap junctions acts alongside cellular interactions achieved by the release of growth factors during embryogenesis. Differences between groups of developing cells may be reflected in, and possibly controlled by, alterations in the selectivity of the gap junctions. It seems likely that gap junctional communication is involved in the control of embryonic patterning rather than phenotypic differentiation.

Membrane topology and quaternary structure of cardiac gap junction ion channels

The membrane topology and quaternary structure of rat cardiac gap junction ion channels containing alpha 1 connexin (i.e. Cx43) have been examined using anti-peptide antibodies directed to seven different sites in the protein sequence, cleavage by an endogenous protease in heart tissue and electron microscopic image analysis of native and protease-cleaved two-dimensional membrane crystals of isolated cardiac gap junctions. Specificity of the peptide antibodies was established using dot immunoblotting, Western immunoblotting, immunofluorescence and immunoelectron microscopy. Based on the folding predicted by hydropathy analysis, five antibodies were directed to sites in cytoplasmic domains and two antibodies were directed to the two extracellular loop domains. Isolated gap junctions could not be labeled by the two extracellular loop antibodies using thin-section immunogold electron microscopy. This is consistent with the known narrowness of the extracellular gap region that presumably precludes penetration of antibody probes. However, cryo-sectioning rendered the extracellular domains accessible for immunolabeling. A cytoplasmic "loop" domain of at least Mr = 5100 (residues (101 to 142) is readily accessible to peptide antibody labeling. The native Mr = 43,000 protein can be protease-cleaved on the cytoplasmic side of the membrane, resulting in an Mr approximately 30,000 membrane-bound fragment. Western immunoblots showed that protease cleavage occurs at the carboxy tail of the protein, and the cleavage site resides between amino acid residues 252-271. Immunoelectron microscopy demonstrated that the Mr approximately 13,000 carboxy-terminal peptide(s) is released after protease cleavage and does not remain attached to the Mr approximately 30,000 membrane-bound fragment via non-covalent interactions. Electron microscopic image analysis of two-dimensional membrane crystals of cardiac gap junctions revealed that the ion channels are formed by a hexagonal arrangement of protein subunits. This quaternary arrangement is not detectably altered by protease cleavage of the alpha 1 polypeptide. Therefore, the Mr approximately 13,000 carboxyterminal domain is not involved in forming the transmembrane ion channel. The similar hexameric architecture of cardiac and liver gap junction connexins indicates conservation in the molecular design of the gap junction channels formed by alpha or beta connexins.

Distribution of mRNAs coding for liver and heart gap junction proteins in the rat central nervous system

The present study examined the distributions of connexin43 mRNA and connexin32 mRNA in the central nervous system (CNS) of the rat by using in situ hybridization histochemistry. These connexins are the best studied gap junction proteins; connexin32 forms direct cell-cell channels in the liver, as does connexin43 in the heart. There was a differential distribution of cells containing connexin32 mRNA compared with the population of cells which contained connexin43 mRNA, thus implying a regional specificity in the expression of connexins in the CNS. Cells containing connexin43 mRNA were uniformly distributed throughout the gray matter of the neuraxis. Several areas had a higher concentration of cells that express connexin43, such as layer IA of the piriform cortex, supraoptic and paraventricular nuclei of the hypothalamus, anterior cortical amygdaloid nucleus, the reticular part of the substantia nigra, lateral habenula, mesencephalic trigeminal nucleus. Purkinje cell layer of the cerebellum, facial nucleus, prepositus hypoglossal nucleus, and dorsal cochlear nucleus. The pattern of connexin43 hybridization and the morphology of connexin43 mRNA containing cells suggest that this gap junction forming protein is found predominantly in astrocytes. Connexin32 mRNA was detected in discrete cell groups of the gray matter that appeared to be neurons, including cells in layer 2 of the neocortex, layer II of the piriform cortex, pyramidal cell layer of the hippocampus, granule and polymorphic cell layers of the dentate gyrus, islands of Calleja, olfactory tubercle, lateral thalamic nuclei, lateral habenula, and Purkinje cell layer of the cerebellar cortex. A large population of cells in white matter tracts that were labelled with the connexin32 riboprobe appeared to be oligodendrocytes. These studies suggest that neurons and glial cells express connexin32 mRNA, but only astrocytes express connexin43 mRNA. Many of the areas in which connexin mRNAs were demonstrated have electrically coupled cells, morphologically distinct gap junction plaques, and/or have immunocytochemically identifiable connexin proteins. These results indicate that cells with mRNAs coding for intercellular channels have a widespread distribution in the mammalian CNS.

Antisera directed against connexin43 peptides react with a 43-kD protein localized to gap junctions in myocardium and other tissues

Rat heart and other organs contain mRNA coding for connexin43, a polypeptide homologous to a gap junction protein from liver (connexin32). To provide direct evidence that connexin43 is a cardiac gap junction protein, we raised rabbit antisera directed against synthetic oligopeptides corresponding to two unique regions of its sequence, amino acids 119-142 and 252-271. Both antisera stained the intercalated disc in myocardium by immunofluorescence but did not react with frozen sections of liver. Immunocytochemistry showed anti-connexin43 staining of the cytoplasmic surface of gap junctions in isolated rat heart membranes but no reactivity with isolated liver gap junctions. Both antisera reacted with a 43-kD polypeptide in isolated rat heart membranes but did not react with rat liver gap junctions by Western blot analysis. In contrast, an antiserum to the conserved, possibly extracellular, sequence of amino acids 164-189 in connexin32 reacted with both liver and heart gap junction proteins on Western blots. These findings support a topological model of connexins with unique cytoplasmic domains but conserved transmembrane and extracellular regions. The connexin43-specific antisera were used by Western blots and immunofluorescence to examine the distribution of connexin43. They demonstrated reactivity consistent with gap junctions between ovarian granulosa cells, smooth muscle cells in uterus and other tissues, fibroblasts in cornea and other tissues, lens and corneal epithelial cells, and renal tubular epithelial cells. Staining with the anti-connexin43 antisera was never observed to colocalize with antibodies to other gap junctional proteins (connexin32 or MP70) in the same junctional plaques. Because of limitations in the resolution of the immunofluorescence, however, we were not able to determine whether individual cells ever simultaneously express more than one connexin type.

Tissue-specific granularity of gap junction cytoplasmic surfaces revealed by rapid-freeze, deep-etch replicas

Previous rapid-freeze, deep-etch replica studies have revealed the differences between heart and liver gap junctions; cytoplasmic surfaces of in situ and phenylmethylsulfonyl fluoride (PMSF)-unproteolyzed isolated cardiac gap junctions (MW 47 kD) have a particulate substructure, which is absent both in the proteolyzed heart junctions (MW 29 kD) and in the liver junctions isolated with PMSF (MW 28 kD). The present deep-etch replica studies of gap junction cytoplasmic surface (CS) membranes in several tissues of rats and mice were performed to examine whether or not this difference between liver and heart is typical of variations in gap junction proteins from tissue to tissue. In surface mucous cells of the stomach, intestinal epithelial cells, and kidney tubule cells, these epithelial gap junctions always showed smooth cytoplasmic surfaces, similar to the liver gap junctions. In contrast, in the atrial myocardium, aortic endothelium, and the ciliary process, cytoplasmic surface membranes of the gap junctions consistently revealed particulate patterns. Close examinations disclosed that those granular structures were not merely attached to the memvrane surface, but they also protruded from the membrane interior as an integral component of gap junction particles. Furthermore, in the pregnant rat uterus at term, cytoplasmic surface membranes of myometrial smooth muscle gap junctions were particulate, but those of endometrial epithelium were smooth. The present observation suggest that tissue specificity exists in cytoplasmic surface structures of gap junctions between the "true" epithelial and the nonepithelial tissues: the nonepithelial gap junctions contain the additional cytoplasmic surface domain that is absent in the gap junctions of "true" epithelial origin.

Isolation and initial characterization of myelin-like membrane fractions from the nerve cord of earthworms (Lumbricus terrestris L)

We report here the isolation of fractions enriched in components of the myelin-like membranes surrounding the giant axons of the earthworm. Lumbricus terrestris L. The composition and purity of the fractions have been assessed using SDS-protein electrophoresis, Western immunoblots, and electron microscopy. Preliminary enzyme assays indicated that the mitochondrial marker, succinate dehydrogenase, has a similar specific activity distribution in earthworm nerve cord and in mouse liver sedimentation velocity fractions, however, the distribution of the total units of activity among the fractions seems to indicate the existence of smaller mitochondria in earthworm nerve cord compared with mouse liver mitochondria. In earthworm nerve cord fractions, Na+/K+ ATPase and Ca2+/Mg2+ ATPase were found to be enriched exclusively in the fraction containing large plasma and myelin-like membranes, while in the mouse liver fractions, the total units of these two enzymes were found to be distributed broadly among fractions. 5'-Nucleotidase activity in the earthworm nerve cord seemed to be restricted to the microsomal fractions (endomembrane network), with a very low activity associated with the large plasma and myelin-like membrane fraction. We have established the presence of keratins or prekeratins in the myelin-like membranes, probably in the form of tonofilaments. However, we could not show that the desmosome-like structures, characteristic of these membranes, are composed of those proteins described for vertebrate epithelial desmosomes.

Immunological characterization of rat cardiac gap junctions: presence of common antigenic determinants in heart of other vertebrate species and in various organs

Antibodies to the following synthetic peptide, SALGKLLDKVQAY, were purified by affinity chromatography and characterized by ELISA and immunoblotting. These antibodies, shown to be specific to the major protein constituent of isolated rat heart junctions: connexin 43, cross-reacted with a homologous protein in immunoreplicas of whole heart fractions of trout, frog, chicken, guinea pig, mouse and rat, suggesting a phylogenic conservation of connexin 43 in vertebrates. By immunoblotting of whole organ fractions it was also demonstrated that these antibodies cross-reacted with major proteins of Mr 32 and 22 kD in rat and mouse liver, of Mr 41 kD in rat cerebellum, of Mr 43 kD in uterus, stomach and kidney of rat, of Mr 46 and 70 kD in rat lens, suggesting that these proteins share common or related epitopes with the synthetic peptide and connexin 43.

Reconstitution of cardiac gap junction channeling activity into liposomes: a functional assay for gap junctions

Cardiac gap junctions were reconstituted into liposomes. To determine if reconstitution resulted in membrane channel formation, we developed an assay for channel function that used a liposome-entrapped peroxidase to detect entry of a substrate into the liposome. The data demonstrate, for the first time, that reconstituted gap junctions from heart are capable of channel-forming activity in artificial membranes.

Junctional complexes and cell polarity in the urinary tubule

In this review, we demonstrate how differentiated membrane domains can be detected in epithelial cells using conventional light and electron microscopy, freeze-fracture electron microscopy and the immuno- and cytochemical detection of membrane components. Using specific examples from the kidney, we show how the polarized insertion of these components into either apical or basolateral plasma membrane regions on either side of the tight junction barrier is related to specific functions of principal and intercalated cells in the collecting duct. In addition, distinct basal and lateral membrane domains have been revealed in some cells that are maintained in the absence of a tight junctional barrier in the plane of the membrane. This suggests that other factors, possibly related to cytoskeletal elements, may be involved in the functional segregation of these membrane areas. We propose that epithelial cell plasma membranes should be subdivided into apical, lateral and basal regions, and that the term "basolateral" may be an oversimplification.

Analysis of the rat liver gap junction protein: clarification of anomalies in its molecular size

The major gap junction polypeptide in most tissues has an apparent molecular mass of 27 kDa with a 47 kDa dimer present in junction-enriched fractions. However, a 54 kDa protein recognized by gap junction-specific antibodies has been reported and a complementary DNA (cDNA) sequence for both human and rat liver gap junctions codes for a 32 kDa protein. In this paper we show that these are all forms of the same gap junction protein that can be observed on SDS-polyacrylamide gels simply by varying the concentration of acrylamide in the gels. A 64 kDa dimer is also obtainable. Antibodies to the gap junction protein or to a synthetic peptide constructed to match the rat liver gap junction amino-terminal sequence recognize all of these forms. Under some conditions a 54 kDa dimer is 'preferred', explaining the presence of this species in whole tissue homogenate Western blots. These results clarify several controversies and indicate that the protein forming the gap junction channel probably undergoes no major post-translational modification as the cDNA sequence codes for a protein of molecular mass 32 kDa and this protein species and its 64 kDa dimer are demonstrable on SDS-polyacrylamide gels under appropriate conditions.

The arthropod gap junction and pseudo-gap junction: isolation and preliminary biochemical analysis

The hepatopancreas of the crayfish, Procambarus clarkii, contains an unusual abundance of gap junctions, suggesting that this tissue might provide an ideal source from which to isolate the arthropod-type of gap junction. A membrane fraction obtained by subcellular fractionation of this organ contained smooth septate junctions, zonulae adhaerentes, gap junctions and pentalaminar membrane structures (pseudo-gap junctions) as determined by electron microscopy. A further enrichment of plasma membranes and gap junctions was achieved by the use of linear sucrose gradients and extraction with 5 mM NaOH. The enrichment of gap junctions correlated with the enrichment of a 31 Kd protein band on polyacrylamide gels. Extraction with greater than or equal to 20 mM NaOH or greater than or equal to 0.5% (w/v) Sarkosyl NL97 resulted in the disruption and/or solubilization of gap junctions. Negative staining revealed a uniform population of 9.6 nm diameter subunits within the gap junctions with an apparent sixfold symmetry. Using antisera to the major gap junctional protein of rat liver (32 Kd) and to the lens membrane protein (MP 26), we failed to detect any homologous antigenic components in the arthropod material by immunoblotting-enriched gap junction fractions or by immunofluorescence on tissue sections. The enrichment of another membrane structure (pseudo-gap junctions), closely resembling a gap junction, correlated with the enrichment of two protein bands, 17 and 16 Kd, on polyacrylamide gels. These structures appeared to have originated from intracellular myelin-like figures in phagolysosomal structures. They could be distinguished from gap junctions on the basis of their thickness, detergent-alkali insolubility, and lack of association with other plasma membrane structures, such as the septate junction. Pseudo-gap junctions may be related to a class of pentalaminar contacts among membranes involved in intracellular fusion in many eukaryotic cell types. We conclude that pseudo-gap junctions and gap junctions are different cellular structures, and that gap junctions from this arthropod tissue are uniquely different from mammalian gap junctions of rat liver in their detergent-alkali solubility, equilibrium density on sucrose gradients, and protein content (antigenic properties).

Roles of cell junctions in gametogenesis and in early embryonic development

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

Connexin43: a protein from rat heart homologous to a gap junction protein from liver

Northern blot analysis of rat heart mRNA probed with a cDNA coding for the principal polypeptide of rat liver gap junctions demonstrated a 3.0-kb band. This band was observed only after hybridization and washing using low stringency conditions; high stringency conditions abolished the hybridization. A rat heart cDNA library was screened with the same cDNA probe under the permissive hybridization conditions, and a single positive clone identified and purified. The clone contained a 220-bp insert, which showed 55% homology to the original cDNA probe near the 5' end. The 220-bp cDNA was used to rescreen a heart cDNA library under high stringency conditions, and three additional cDNAs that together spanned 2,768 bp were isolated. This composite cDNA contained a single 1,146-bp open reading frame coding for a predicted polypeptide of 382 amino acids with a molecular mass of 43,036 D. Northern analysis of various rat tissues using this heart cDNA as probe showed hybridization to 3.0-kb bands in RNA isolated from heart, ovary, uterus, kidney, and lens epithelium. Comparisons of the predicted amino acid sequences for the two gap junction proteins isolated from heart and liver showed two regions of high homology (58 and 42%), and other regions of little or no homology. A model is presented which indicates that the conserved sequences correspond to transmembrane and extracellular regions of the junctional molecules, while the nonconserved sequences correspond to cytoplasmic regions. Since it has been shown previously that the original cDNA isolated from liver recognizes mRNAs in stomach, kidney, and brain, and it is shown here that the cDNA isolated from heart recognizes mRNAs in ovary, uterus, lens epithelium, and kidney, a nomenclature is proposed which avoids categorization by organ of origin. In this nomenclature, the homologous proteins in gap junctions would be called connexins, each distinguished by its predicted molecular mass in kilodaltons. The gap junction protein isolated from liver would then be called connexin32; from heart, connexin43.

Cloning and characterization of human and rat liver cDNAs coding for a gap junction protein

An extended synthetic oligonucleotide (58-mer) has been used to identify and characterize a human liver gap junction cDNA. The cDNA is 1,574 bases long and contains the entire coding region for a gap junction protein. In vitro translation of the RNA products of this cDNA is consistent with it coding for a 32,022-D protein. Southern blot analysis indicates that the gap junction gene is present as a single copy, and that it can be detected in a variety of organisms using the human liver cDNA as a probe. The human cDNA has been used to screen a rat liver cDNA library, and a rat liver junction cDNA clone has been isolated. The rat liver clone is 1,127 bases in length, and it has strong sequence homology to the human cDNA in the protein-coding region, but less extensive homology in the 3'-untranslated region.

Molecular cloning of cDNA for rat liver gap junction protein

An affinity-purified antibody directed against the 27-kD protein associated with isolated rat liver gap junctions was produced. Light and electron microscopic immunocytochemistry showed that this antigen was localized specifically to the cytoplasmic surfaces of gap junctions. The antibody was used to select cDNA from a rat liver library in the expression vector lambda gt11. The largest cDNA selected contained 1,494 bp and coded for a protein with a calculated molecular mass of 32,007 daltons. Northern blot analysis indicated that brain, kidney, and stomach express an mRNA with similar size and homology to that expressed in liver, but that heart and lens express differently sized, less homologous mRNA.