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Results for “Gap Junction beta-1 Protein”

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At least 19 recordsLinked to original sources
PubMed1995

The topogenic fate of the polytopic transmembrane proteins, synaptophysin and connexin, is determined by their membrane-spanning domains.

The synaptophysins and connexins are polytopic transmembrane proteins of similar secondary structure that accumulate as multiple homo-oligomers in specialized membrane regions, the presynaptic transmitter vesicles or gap junctions. Transfection and expression of the respective genes in cultured epithelial cells results in the de novo formation of either small cytoplasmic, synaptophysin-rich vesicles, or functional gap junctions consisting of clustered connexin molecules. To examine the molecular requirements for the specific enrichment and topogenesis of both types of molecule, chimeric cDNAs were constructed composed of different parts of the rat synaptophysin and rat liver connexin32 genes. Expression of the encoded chimeric polypeptides in hepatocellular carcinoma-derived cells showed that only chimeras with all four transmembrane domains from either parent molecule were delivered to their specific destination. In contrast, chimeras with transmembrane domains from both connexin32 and synaptophysin were always retained in the endoplasmic reticulum. The topogenic nature of the transmembrane domains was further demonstrated by deletion mutagenesis, indicating that removal of cytoplasmic end domains or intravesicular loops does not abolish targeting. On the other hand, excision of individual transmembrane domains or introduction of point mutations in transmembrane segments resulted in retention in the endoplasmic reticulum.

Animals↗
PubMed1995

Molecular anatomy and genetics of myelin proteins in the peripheral nervous system.

Myelin contains a number of proteins, the major examples of which are protein zero (Po), P2 protein, peripheral myelin protein 22 (PMP22), myelin basic proteins (MBPs), myelin-associated glycoprotein (MAG) and the recently described connexin 32 (Cx32). This list is probably still incomplete. The localisation and possible functions of these proteins are reviewed. In the past few years a number of inherited demyelinating neuropathies in mice and the human have been shown to be due to mutations affecting the genes PMP22, Po and Cx32 so that it has become possible to characterise the molecular pathology of the majority of these disorders. This has provided important insights into the relationships between the structure of myelin and the function of its constituent proteins.

Animals↗
PubMed1995

From the syndrome of Charcot, Marie and Tooth to disorders of peripheral myelin proteins.

The description of the peroneal muscular atrophy syndrome in 1886 by Charcot, Marie and Tooth was followed by an era of nosological confusion. This was partly clarified by the advent of nerve conduction studies and the definition of the most common, but heterogeneous, disorders underlying this syndrome, hereditary motor and sensory neuropathies (HMSN) types I and II. The classification of HMSN is now changing as a result of the identification of underlying mutations in genes encoding myelin proteins. Abnormalities of peripheral myelin protein 22 (PMP-22) account for dominantly inherited HMSN type I in approximately 90% of families. The commonest genetic defect is a duplication of this gene and the surrounding region of chromosome 17, although point mutations also occur. A deletion of the same region causes hereditary neuropathy with liability to pressure palsies. Point mutations of the P0 gene cause HMSN I in a small number of families. The X-linked type of HMSN is associated with defects of the connexin 32 gene, which encodes a gap junction protein. These molecular genetic advances can be translated into clinical practice, leading to improved diagnosis and genetic counselling.

Charcot-Marie-Tooth Disease↗
PubMed1995

Physical characterization of gap junction membrane connexons (hemi-channels) isolated from rat liver.

Enriched subcellular fractions of double membrane gap junctions (plaques) from rat livers were treated under reducing conditions with high salt and non-ionic detergent concentrations at high pH to obtain a preparation of structural 80-90 A complexes of oligomers (connexons). The isolated oligomers were chromatographically purified, and subsequently characterized immunologically, morphologically by electron microscopy, hydrodynamically by gel filtration and ultracentrifugation, spectroscopically by circular dichroism, and chemically via cross-linking studies. The physical characteristics of these isolated gap junction complexes were compared to those of native membrane-bound gap junctions in rat liver. These analyses indicate that the isolated complex (connexon) principally contains a hexameric arrangement of gap junction protein to form a single membrane hemi-channel.

Animals↗
PubMed1995

Functional analysis of selective interactions among rodent connexins.

One consequence of the diversity in gap junction structural proteins is that cells expressing different connexins may come into contact and form intercellular channels that are mixed in connexin content. We have systematically examined the ability of adjacent cells expressing different connexins to communicate, and found that all connexins exhibit specificity in their interactions. Two extreme examples of selectivity were observed. Connexin40 (Cx40) was highly restricted in its ability to make heterotypic channels, functionally interacting with Cx37, but failing to do so when paired with Cx26, Cx32, Cx43, Cx46, and Cx50. In contrast, Cx46 interacted well with all connexins tested except Cx40. To explore the molecular basis of connexin compatibility and voltage gating, we utilized a chimera consisting of Cx32 from the N-terminus to the second transmembrane domain, fused to Cx43 from the middle cytoplasmic loop to the C-terminus. The chimeric connexin behaved like Cx43 with regard to selectivity and like Cx32 with regard to voltage dependence. Taken together, these results demonstrate that the second but not the first extracellular domain affects compatibility, whereas voltage gating is strongly influenced by sequences between the N-terminus and the second transmembrane domain.

Animals↗
PubMed1995

New point mutations and deletions of the connexin 32 gene in X-linked Charcot-Marie-Tooth neuropathy.

The purpose of this study was the identification of new mutations of the connexin 32 (CX32) gene in CMTX families. We report six new mutations of the CX32 gene including two medium sized (29 and 18 bp) deletions. The clinical phenotype is consistent with CMT peripheral neuropathy in all patients. Four families show both male and female patients, with more severe symptoms in males. The disease is asymptomatic in females in two families. The clinical deficit in CMTX families Nos 1, 2 and 4 with missense mutations of the CX32 gene was mild or moderate. Severe weakness of the feet and hands was present in CMTX family No. 5 with a G insertion and family No. 6 with a 29 bp deletion in the carboxyl terminal region of the CX32 gene. Most likely the severe clinical impact in those families was related to frame shift and premature termination of the protein.

Adolescent↗
PubMed1995

Reappearance and long-term maintenance of connexin32 in proliferated adult rat hepatocytes: use of serum-free L-15 medium supplemented with EGF and DMSO.

Intercellular communication, especially gap junctional communication, is thought to be one of the highly differentiated functions of hepatocytes. In primary cultures of rat hepatocytes, it has been considered that the maintenance and the reinduction of differentiated functions is very difficult. In the present study, we succeeded in inducing the gap junctional protein connexin32 (Cx32) in adult rat hepatocytes cultured in serum-free L-15 medium supplemented with epidermal growth factor (EGF) and dimethylsulfoxide (DMSO). When the hepatocytes were cultured in L-15 medium supplemented with 20 mM NaHCO3 and 10 ng/ml EGF in a 5% CO2:95% air incubator, the cells proliferated. Fluorescence immunocytochemistry showed spots immunoreactive to Cx32 on the cell membranes between adjacent cells until day 3, but only a few Cx32-positive spots were found after day 4. Western and northern blot analyses also showed that the amounts of both the protein and mRNA of Cx32 in the cells decreased with time in culture. However, when the cells were treated with 2% DMSO from day 4, the immunoreactive spots reappeared on the cell membranes from day 6 and both their number and intensity gradually increased. The reappearance of Cx32 was accompanied by increases in both the protein and mRNA of Cx32. Furthermore, the expression of Cx32 was well maintained, together with extensive gap junctional intercellular communication, for more than 4 weeks. In addition, ultrastructurally, many gap junctional structures were observed between the hepatocytes, and the antibodies to Cx32 were shown to bind to those structures. This culture system may be useful for studies of the reconstruction of the gap junctional structure, the intracellular pathways of the proteins, and the regulation of synthesis and processing in differentiated hepatocytes.

Animals↗
PubMed1995

Immunohistological detection of gap junctions in human lymphoid tissue: connexin43 in follicular dendritic and lymphoendothelial cells.

We investigated the expression of gap junction connexins26, -32, and -43 in normal, reactive, and diseased human lymphoid tissue with single and double immunolabeling and confocal laser scanning microscopy. In all tissues, connexin43 positivity was detected in follicular dendritic cells positive for CD21 and CD35 antigens, around lymphoendothelial cells moderately positive for Factor VIII, CD31 and cathepsin-D antigens; and somewhat in vascular endothelia including high endothelial venules strongly positive for Factor VIII and CD31 antigens. The ultrastructural hallmark of gap junctions, pentalaminar structures with appropriate spacing, was found in follicular dendritic cell processes. Connexin43 was also detected between smooth muscle and stromal cells of the gut, in capsular fibroblasts, and in tonsil epithelium. Neither connexin32 nor -26 was revealed, except for connexin26 in the tonsil epithelium. In follicular dendritic cells, connexin43 co-localized closely with the desmosomal proteins desmoplakin and desmoglein, suggesting that cell adherence has a role in gap junction formation. Most connexin43 was observed in sinus lining cells of lymph nodes involved in malignancies and in follicular dendritic cells in the light zone of germinal centers where maturing but still proliferating lymphocytes are situated. In the light of their distribution, gap junctions may play a part in regulating the growth of germinal centers and in integrating activating or controlling signals in follicular dendritic and sinus lining cell networks. Because connexin43 is the connexin of stromal cells, finding it in follicular dendritic cells in consistent with the proposal that these cells originate from resident stromal cells.

Animals↗
PubMed1995

Gap junctions and cell polarity: connexin32 and connexin43 expressed in polarized thyroid epithelial cells assemble into separate gap junctions, which are located in distinct regions of the lateral plasma membrane domain.

Epithelial cells of the thyroid gland present an uncommon connexin expression pattern, they coexpress connexin32 and connexin43. In the present work, we have analyzed the membrane distribution of these two connexins to determine: (i) whether they co-assemble in the same gap junctions or form separate gap junctions; and (ii) whether their location is somehow related to the thyroid cell polarity. Immunofluorescence analyses of the localization of the two connexins in thyroid tissue sections revealed that connexin32 and connexin43 are located in different regions of the plasma membrane. We further analyzed the location of each of the two connexins with regard to that of the tight junction-associated protein, ZO1. Laser scanning confocal microscope observations of connexin32 or connexin43 and ZO1 double-immunolabelled thyroid cells, gave evidence for a separate localization of gap junctions made of each of these two connexins. Connexin32 gap junctions appeared as fluorescent spots scattered over the lateral membrane domain, while connexin43 gap junctions formed a meshed network superimposable with that of tight junctions in the subapical region of the cells. Western blot analyses of the distribution of connexins in thyroid plasma membrane subfractions obtained by ultracentrifugation on a sucrose gradient led to the identification of membrane sub-populations enriched in either connexin32 gap junctions or connexin43 gap junctions. Connexin32 gap junctions and connexin43 gap junctions were found to differ in their resistance to solubilization by N-lauroylsarcosine. Increasing concentrations of this detergent from 0.12% to 0.42% caused a progressive solubilization of connexin43 while connexin32 remained membrane-bound.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

Immunohistochemistry of gap junctions in normal and diseased gastric mucosa of humans.

BACKGROUND & AIMS: Intercellular communication through gap junctions has been proposed to be an important mechanism for the maintenance of tissue homeostasis. However, few studies have considered the role of gap junctions in gastric mucosa. The purpose of this study was to evaluate the distribution of gap junction-specific proteins in normal and diseased gastric mucosa of humans. METHODS: Biopsy specimens were obtained endoscopically. Immunohistochemical staining was performed by standard immunoperoxidase techniques using an anti-connexin 32 monoclonal antibody. RESULTS: In normal gastric mucosa, connexin 32 was present chiefly in the foveolar cells and in a decreasing staining gradient extending down to the necks of the glands. Connexin 32 also was observed in the epithelial cells of atrophic mucosa in a pattern similar to that observed in normal controls. Conversely, the majority of the foveolar cells adjacent to erosions had reduced or absent staining. Connexin 32 also was reduced significantly or absent from metaplastic epithelial cells. No malignant cells from patients with carcinoma contained detectable connexin 32. CONCLUSIONS: Intercellular communication likely is impaired in precancerous or paracancerous lesions of the stomach. Abnormal intercellular communication thus may play an important role in the progression from mucosal injury to intestinal metaplasia and/or gastric carcinoma.

Adult↗
PubMed1994

Modulation of connexins during differentiation of oval cells into hepatocytes.

The connexins are a family of related gap-junction proteins, implicated in embryonic development, cell growth control, and cellular differentiation. To identify connexins involved in liver cell differentiation, both in vivo and in vitro systems were employed to study expression of connexins 26, 32, and 43. Northern blot analysis and in situ hybridization were used to measure the levels of connexin expression and cellular localization of the transcripts, respectively. Normal liver expressed high connexin 32, low connexin 26, and barely detectable connexin 43. In vivo proliferation and differentiation of oval cells was at first accompanied by increased connexin 43 and decreased connexin 32 expression; later as the oval cells differentiated into hepatocytes, connexin 43 disappeared and connexin 32 increased to control levels. In situ hybridization showed that both oval cells and bile duct epithelial cells, but not hepatocytes, expressed connexin 43. A switch from connexin 43 to connexin 32 expression was observed following in vitro transformation and differentiation of rat liver epithelial cells toward the hepatocytic lineage. These results suggest that early progenitor cells in the liver express connexin 43 and a switch from connexin 43 to connexin 32 may signal commitment to hepatocytic differentiation.

Animals↗
PubMed1995

Use of alternate promoters for tissue-specific expression of the gene coding for connexin32.

The promoter of rat connexin32 (Cx32), the gap junction protein found in liver, was studied in transgenic mice. Cx32 transgenes, containing 2.5-kb of sequence upstream from the promoter, exon I, the entire 6.1-kb intron and the beginning of the coding sequence linked to the gene encoding luciferase (Luc), were found to be expressed in mouse in the same tissue-specific manner as previously reported for Cx32. Another construct lacking the promoter, but retaining 1.8 kb from the 3' end of the intron, was found to be expressed specifically in the nervous system. This result suggested that a second promoter, different from that used in liver, functions in nervous tissue. The use of this promoter in normal rats was corroborated by sequence analysis of reverse-transcribed PCR products obtained from rat nervous tissue RNA. The second promoter drives the synthesis of a second Cx32 mRNA species that is processed to remove a small 345-bp intron that shares its acceptor splice site with the large intron. This finding could have implications for the genetic basis of the X-linked form of Charcot-Marie-Tooth disease (CMT-X) in those patients that do not exhibit mutations in the Cx32-coding region.

Animals↗
PubMed1995

Heterotypic gap junction channels (connexin26-connexin32) violate the paradigm of unitary conductance.

Human HeLa cells transfected with mouse DNA coding for connexin26 (Cx26) or connexin32 (Cx32) were used to examine the properties of heterotypic Cx26-Cx32 gap junction channels. Intercellular current flow was examined in induced cell pairs by means of the dual voltage-clamp method. We found that Cx26-Cx32 channels exhibit voltage-dependent conductances, gamma j: gamma j(main state) increases with increasing positivity at the cytoplasmic aspect of the Cx26 connexon and decreases with increasing negativity (slope: 32 pS/100 mV; gamma j(main state) reaches 48 pS as Vj approaches 0 mV); gamma j(residual state) with a similar Vj-dependence is present when the cytoplasmic end of Cx26 connexon is positive, but absent when it is negative. The single channel data provide an explanation for the asymmetric relationships between the gap junction conductance, gj, and Vj. The results are consistent with the notion that docking of two connexons co-determines the biophysical properties of a gap junction channel.

Animals↗
PubMed1995

Inhibition of gap junctional intercellular communication by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rat hepatocytes.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent rodent hepatic tumor promoter. Unlike observations with the majority of tumor promoting chemicals studied to date, most investigations have failed to demonstrate down-regulation of gap junctional intercellular communication (GJIC) in cultured cells by TCDD. The present study examined the effect of TCDD on GJIC in rat hepatocytes in primary culture. At non-cytolethal doses TCDD inhibited GJIC in a time- (1, 4, 24 and 48 h) and concentration (1 x 10(-8) - 1 x 10(-14) M)-dependent manner. This inhibition occurred within 4 h of treatment at doses of 1 x 10(-8) - 1 x 10(-12) M TCDD and persisted for up to 48 h, despite removal of TCDD. Treatment of rat hepatocytes with TCDD resulted in a decrease in hepatocyte connexin 32 mRNA, but had no apparent effect on connexin 26 mRNA. Co-incubation of rat hepatocytes with TCDD and alpha-napthoflavone abolished down-regulation of GJIC by TCDD. Similarly, co-treatment with a cAMP analog (8-bromoadenosine 3',5'-cyclic monophosphate) prevented down-regulation of GJIC by TCDD. The results of this investigation demonstrated, for the first time, that TCDD inhibits GJIC in the in vivo target of its tumor promoting effect and that this effect may, in part, be mediated through the Ah receptor. In addition, this study showed that inhibition of GJIC by TCDD may be due to transcriptional down-regulation or stability of the connexin 32 gap junction mRNA.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMedDate not supplied

Consortium fine localization of X-linked Charcot-Marie-Tooth disease (CMTX1): additional support that connexin32 is the defect in CMTX1.

Charcot-Marie-Tooth (CMT) disease is the most common form of inherited motor and sensory neuropathy. X-linked CMT (CMTX1) has been localized to the pericentric region of the X chromosome. Recently, mutations have been defined in the connexin32 gene that cosegregate with the CMTX1 phenotype in several families. The present paper presents the results of an international consortium to fine map the gene for CMTX1 to a small segment of Xq12-13. The linkage data, together with the molecular genetic studies, support the hypothesis that connexin32 is the genetic defect in CMTX1.

Charcot-Marie-Tooth Disease↗