Analysis of vertebrate gap junction protein

Loss of Ephaptic Contacts in the Murine Thalamus during Osmotic Demyelination Syndrome

BACKGROUND AND AIM

A murine model mimicking osmotic demyelination syndrome (ODS) revealed with histology in the relay posterolateral (VPL) and ventral posteromedial (VPM) thalamic nuclei adjoined nerve cell bodies in chronic hyponatremia, amongst the damaged 12 h and 48 h after reinstatement of osmolality. This report aims to verify and complement with ultrastructure other neurophysiology, immunohistochemistry, and molecular biochemistry data to assess the connexin-36 protein, as part of those hinted close contacts.This ODS investigation included four groups of mice: Sham (NN; n = 13), hyponatremic (HN; n = 11), those sacrificed 12 h after a fast restoration of normal natremia (ODS12h; n = 6) and mice sacrificed 48 h afterward, or ODS48 h (n = 9). Out of these, thalamic zones samples included NN (n = 2), HN (n = 2), ODS12h (n = 3) and ODS48h (n = 3).

RESULTS

Ultrastructure illustrated junctions between nerve cell bodies that were immunolabeled with connexin36 (Cx36) with light microscopy and Western blots. These cell's junctions were reminiscent of low resistance junctions characterized in other regions of the CNS with electrophysiology. Contiguous neurons showed neurolemma contacts in intact and damaged tissues according to their location in the ODS zones, at 12 h and 48 h post correction along with other demyelinating alterations. Neurons and ephaptic contact measurements indicated the highest alterations, including nerve cell necrosis in the ODS epicenter and damages decreased toward the outskirts of the demyelinated zone.

CONCLUSION

Ephapses contained C × 36between intact or ODS injured neurons in the thalamus appeared to be resilient beyond the core degraded tissue injuries. These could maintain intercellular ionic and metabolite exchanges between these lesser injured regions and, thus, would partake to some brain plasticity repairs.

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.

Sequence diversity of gap junction proteins

This paper summarizes our understanding of the molecular organization of gap junction proteins. There appear to be overall similarities in the organization of heart and liver junctions in terms of general domains, even though the molecular sizes of the two proteins are quite different. Sequence data on the amino-terminal regions of these two proteins show 43% of the residues to be identical and 25% more to be homologous. The major intrinsic protein of lens (MIP), believed by many to be the lens-fibre junction protein, does not show such sequence homology with the known portions of junction proteins from either heart or liver. Yet the sequence of MIP, which is completely known, suggests a conformation for this molecule quite compatible with a junctional role. It thus appears that molecules potentially involved in junction formation will prove to form a rather diverse family, with special characteristics of organ-specific molecules that may well be related to their function.

In vitro expression of the 15 kDa subunit of the mediatophore and functional reconstitution of acetylcholine release

The mediatophore is a presynaptic oligomeric protein purified from the presynaptic plasma membrane of Torpedo synaptosomes on the basis of its ability to mediate a calcium-dependent acetylcholine release when solubilized and reconstituted into proteoliposomes. We investigated the ACh translocating activity of the 15 kDa proteolipid subunit of the mediatophore when expressed in Xenopus oocytes and reconstituted into proteoliposomes loaded with ACh. 1. A calcium-dependent ACh translocation was observed when oocytes were injected with polyadenylated mRNAs extracted from the electric lobe of the Torpedo brain or with an in vitro transcribed RNA encoding the 15 kDa subunit. 2. No release response was obtained when oocytes were non-injected or injected with Torpedo liver mRNAs. 3. This ACh translocation mechanism showed calcium-dependent activation and desensitisation and was inhibited by cetiedil, sharing these properties with the release of ACh observed at the synapse. 4. The ACh translocating activity of an N terminal deleted mediatophore 15 kDa subunit was strongly reduced and the deleted proteolipid appeared less sensitive to the action of cetiedil (alpha-cyclohexyl-alpha-(3-thienyl)-acetate of perhydroazepinyl-alpha-ethyl citrate monohydrate). 5. A significant ACh release response was observed when the 15 kDa proteolipid of the H(+)-ATPase from bovine chromaffin granules was tested. 6. These results show that this ACh translocating activity could be induced in the oocyte membranes by the expression of the 15 kDa subunit alone.

Connexins and the vacuolar proteolipid-like 16-kDa protein are not directly associated with each other but may be components of similar or the same gap junctional complexes

Gap junction preparations made from mouse liver plasma membranes by alkali extraction contain variable proportions of connexins (Cx32 and Cx26) and the 16-kDa protein which is closely related or may be identical to the 16-kDa proteolipid (subunit c) of the vacuolar H(+)-ATPase and the mediatophore complex. The absence of a stoichiometric relationship suggests that connexins and the 16-kDa protein are not subunits of the same channel complex, but analysis of alkali preparations by isopycnic centrifugation shows both types of protein are in membrane structures of the same buoyant density. Electron microscopic analysis of alkali preparations shows a homogeneous population of gap junctions of uniform morphology and width, suggesting the proteins are in the same or similar structures. The structures containing connexins and the 16-kDa protein can be separated by treatment of the plasma membranes with Triton X-100. After such treatment, the connexins remain associated with dense cellular or extracellular material and the gap junctional structures, after further extraction with N-lauroyl sarcosine and urea, contain only the 16-kDa protein. These detergent-extracted gap junctions are thinner (14.1 nm) than those in alkali preparations (18.4 nm).

Connexon integrity is maintained by non-covalent bonds: intramolecular disulfide bonds link the extracellular domains in rat connexin-43

Rat heart connexin-43 (RCx43) has been isolated using a modified procedure that is rapid and can be used with fresh or frozen hearts. When RCx43 is isolated in the presence of the alkylating reagent iodoacetamide, no intermolecular disulfide bonds are found. However, the alkylated RCx43 does have at least one intramolecular disulfide bond. By using site directed antibodies and proteolytic cleavage the location of the intramolecular disulfide bonding is shown between the two extracellular loops of RCx43.

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.

A 15 kDa proteolipid found in mediatophore preparations from Torpedo electric organ presents high sequence homology with the bovine chromaffin granule protonophore

Upon SDS PAGE of isolated mediatophore, an acetylcholine-translocating protein, a doublet at 15 kDa was identified. Amino acid sequencing after CNBr cleavage gave a 17 residue-long peptide completely homologous with a sequence of the proton-translocating proteolipid from bovine chromaffin granules. A 51-mer oligodeoxynucleotide corresponding to this sequence was used to screen a library of electric lobe cDNAs constructed in lambda Zap II. A positive recombinant clone was isolated and found to encode the complete sequence of a 15.5 kDa protein highly homologous to the bovine chromaffin or yeast vacuolar ATPase proteolipid. In vitro translation of sense RNA transcripts of the clone indeed yielded a single 15 kDa proteolipid. Northern blot analysis showed that the 1.3 kb mRNA encoding this protein is significantly expressed in nervous tissues but not in electric organ or liver of Torpedo marmorata.

Junctional communication and cellular differentiation

Gap junctions provide pathways of direct cell to cell communication in the tissues of metazoan animals. Cells joined by gap junctions share their small ions and molecules but can maintain distinctive activities through expression of different macromolecules which are too large to pass through the junctions. The junctional channels are made of a tissue invariant, evolutionarily conserved 16-18 k protein but the formation and maintenance of active coupling also requires one or more connexins, a family of tissue-specific proteins ranging in size from 21 k to 70 k. Junctions can be isolated as complexes containing both types of protein by mild procedures using high pH but the connexins can be removed by detergent, urea and protease treatment without destroying the characteristic junctional-morphology of hexagonally packed channels in the double membrane structures. There is also some evidence for the participation in the complex of tissue-specific proteoglycans which perhaps interact with the tissue-specific connexins and account for specificity of junction formation. Such specificity in mixed cultures leads to the production of communication compartments, groups of cells joined by junctions but separated by reduced trans-boundary coupling from cells in adjacent compartments. Compartmentation also occurs in vivo resulting in specific patterns of junctional communication which have been mapped in most detail in mouse skin. These mapping data and the changes which are associated with abnormal proliferation have lead to new ideas on intercellular control.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell-to-cell channel conductance during loss of gap junctional coupling in pairs of pancreatic acinar and Chinese hamster ovary cells

Electrical coupling, mediated by gap junctional channels connecting pairs of murine pancreatic acinar cells and Chinese hamster ovary (CHO) cells, was studied using the double whole cell patch clamp technique. Two approaches were used to reduce the junctional conductance (gj) in order to study gj at the single channel level. During spontaneous uncoupling, single channel conductances of 130 pS and 27 pS could be characterized using freshly isolated acinar cells. In most experiments, stepwise conductances could not be discriminated while gj decreased gradually below 10 pS. In CHO cell pairs, discrete junctional channel conductances of 120 pS, 70 pS, 50 pS, 37 pS and 22 pS were identified. Exposure of pancreatic acinar cell pairs to 0.4 mM octanol resulted in rapid and reversible uncoupling. Discrete junctional conductance steps could not clearly be identified down to a gj of about 3 pS. The influence of the composition of the pipette solution on spontaneous uncoupling was investigated. Addition of 5 mM ATP and 0.1 mM cAMP to the pipette electrolyte was sufficient to stabilize coupling in the experimental time range of up to 1 h. Different mechanisms of uncoupling, including an increase of flickering in the channel open state, and modulation of the number of channels exhibiting different conductance or subconductance states are discussed.

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.

Tissue and species conservation of the vertebrate and arthropod forms of the low molecular weight (16-18000) proteins of gap junctions

Gap junctions have been isolated from four murine tissues, from rat and Xenopus laevis liver, and from Nephrops norvegicus (Norway lobster) hepatopancreas. The preparations of gap junctions from each vertebrate tissue contain a single major protein, Mr 16,000, and those from Nephrops hepatopancreas a protein, Mr 18,000. Immunocytochemical studies using affinity-purified antibodies raised against gap junctions from Nephrops show the junctional origin of the 18k protein. Immunological studies using Western blotting and biochemical studies using tryptic peptide mapping show no significant differences between the 16k junctional proteins of mouse and hence provide no evidence of tissue variation. These studies also suggest that the mouse, rat, and Xenopus 16k proteins and the Nephrops 18k protein share some common structural features.

Two homologous protein components of hepatic gap junctions

Gap junctions consist of closely packed pairs of transmembrane channels, the connexons, through which materials of low relative molecular mass diffuse from the cell to neighbouring cells. In liver, connexons consist of six protein subunits which, until now, were believed to be identical. However, besides the major polypeptide of relative molecular mass (Mr) 28,000 (and see refs 4 and 6), a component of Mr 21,000 (21K) has been repeatedly observed in liver. The amino-terminal sequence (18 residues) of this less abundant protein shows that it is related to, but distinct from, the Mr 28K protein. Immuno-staining and immuno-precipitation show both proteins to be in the same gap junctional plaques. Thus, it seems that hepatic gap junction channels (and by extension possibly others) are composed of two (or more) homologous proteins.

Characterization of a rat liver epithelial cell line to detect inhibitors of metabolic cooperation

A normal rat liver epithelial cell line, with phenotype characteristics of "oval" cells (WB-F344), was examined for its ability to perform gap-junctional intercellular communication as measured by metabolic cooperation. To test for gap-junctional intercellular communication, 6-thioguanine-sensitive cells were cocultivated with 6-thioguanine-resistant cells. It was found that the recovery of 6-thioguanine-resistant cells depended on the densities of the 6-thioguanine-sensitive cells. Higher densities of 6-thioguanine-sensitive cells reduced the recovery of 6-thioguanine-resistant cells. These observations demonstrate that rat liver epithelial cells could metabolically cooperate, implying they could perform gap-junctional intercellular communication. Two tumor-promoting organochlorine pesticides, aldrin and dieldrin, were potent inhibitors of metabolic cooperation for these cells, but 12-0-tetradecanoyl-phorbol-13-acetate and teleocidin, known mouse skin tumor promoters, were not significantly effective in inhibiting metabolic cooperation. The results suggest that these cells might provide the basis for an in vitro assay specifically to study liver tumor promoters.

The cardiac gap junction protein (Mr 47,000) has a tissue-specific cytoplasmic domain of Mr 17,000 at its carboxy-terminus

The molecular weight of the heart gap junctional protein subunit was, until recently, believed to be about Mr 28,000-30,000, similar to that of other previously characterized gap junctional proteins. A larger polypeptide of about Mr 44,000-47,000, which undergoes proteolysis during isolation, has recently been proposed as the form of the heart junction protein in vivo. We show here that this entity has the same amino-terminal sequence as the previously characterized Mr 29,000-30,000 component. Thus, the cardiac junctional protein has, at its carboxy-terminus, cytoplasmic domain of Mr 17,000; this domain is absent in the liver protein. These observations provide further evidence that gap junction proteins form a highly diversified family.

Identification of a rat liver cDNA and mRNA coding for the 28 kDa gap junction protein

By screening of a rat liver cDNA library with complex and deoxyinosine containing oligonucleotide probes a cDNA clone was isolated and shown by sequencing to code for the amino-terminal half of the rat liver 28 kDa gap junction protein. The insert hybridized to a 1.9 kb species from rat and mouse liver poly(A)+ RNA in Northern blot analysis. In embryonic mouse hepatocytes the amount of the 1.9 kb mRNA increased 3-fold between 24 and 96 h in culture. This correlates with the previously described increase of the 28 kDa gap junction protein under these conditions.

Cell-cell interaction and carcinogenesis

Before carcinogenic stimuli are introduced, a potentially cancerous cell functions normally; growth is controlled and the cell exists in complete harmony with that of surrounding cells in a given tissue. During the process of multi-stage carcinogenesis, such harmony is destroyed and a presumptive initiated cell accomplishes a clonal expansion, neglecting the growth pattern of surrounding normal cells. Recent results from our own laboratory and others strongly suggest that blocked gap-junctional intercellular communication is an important determinant during the process of tumor promotion.