Immunohistochemical analysis of rat liver using a monoclonal antibody (HAM8) against gap junction

Morphological and functional changes of the rat parotid glandular cells by clipping and reopening the parotid duct, using HAM8 antibody

The purpose of this experiment is to examine the proliferative process of rat acinar cells after parotid duct ligation and reopening. Two experimental groups were observed. The first group was killed from 0 to 14 days after the duct ligation. In the second group, the duct was clipped for 14 days, and it was reopened. Following a period of from 2 to 28 days after removal of the clip, the glands were removed to perform a histological analysis, including hematoxylin-eosin (HE), immunofluorescent staining using HAM8 antibody, which recognizes connexin 32, and transmission electron microscopy (TEM). In the experimental gland from the 1st group at 6 days after ligation (I-6D), the acinar cells disappeared. In the tissue from the 2nd group 8 days after reopening (II-8D), newly formed acinar cells were found again. Lobular structure of the parotid glands recovered in the II-21D. HAM8 signals were observed between normal acinar cells, while they declined in the tissue from I-1D, and they were not observed in the I-2D. HAM8 signals were first observed in the II-25D and then subsequently returned to normal levels in the II-28D. These results suggest that the intercellular communication and functional recovery was not complete 25 days after reopening of the duct.In conclusion, the recovery of the acinar structure was recognized during an extended period of duct ligation, however, a time lag between the morphological and functional recovery was found to exist.

Differential potency of dominant negative connexin43 mutants in oculodentodigital dysplasia

Oculodentodigital dysplasia (ODDD) is a congenital autosomal dominant disorder with phenotypic variability, which has been associated with mutations in the GJA1 gene encoding connexin43 (Cx43). Given that Cx43 mutants are thought to be equally co-expressed with wild-type Cx43 in ODDD patients, it is imperative to examine the consequence of these mutants in model systems that reflect this molar ratio. To that end, we used differential fluorescent protein tagging of mutant and wild-type Cx43 to quantitatively monitor the ratio of mutant/wild-type within the same putative gap junction plaques and co-immunoprecipitation to determine if the mutants interact with wild-type Cx43. Together the fluorescence-based assay was combined with patch clamp analysis to assess the dominant negative potency of Cx43 mutants. Our results revealed that the ODDD-linked Cx43 mutants, G21R and G138R, as well as amino terminus green fluorescent protein-tagged Cx43, were able to co-localize with wild-type Cx43 at the gap junction plaque-like structures and to co-immunoprecipitate with wild-type Cx43. All Cx43 mutants demonstrated dominant negative action on gap junctional conductance of wild-type Cx43 but not that of Cx32. More interestingly, these Cx43 mutants demonstrated different potencies in inhibiting the function of wild-type Cx43 with the G21R mutant being two times more potent than the G138R mutant. The potency difference in the dominant negative properties of ODDD-linked Cx43 mutants may have clinical implications for the various symptoms and disease severity observed in ODDD patients.

Defective gap junctional intercellular communication in the carcinogenic process

Gap junctions are membrane structures made of intercellular channels which permit the diffusion from cytoplasm to cytoplasm of small hydrophilic molecules. Nearly 40 years ago, the loss of functional gap junctions has been described in cancer cells and led to the hypothesis that such type of intercellular communication is involved in the carcinogenesis process. From this time, a lot of data has been accumulated confirming that gap junctions are frequently decreased or absent in cancer cells whatever their tissue and species origins. Here, we review such data by insisting on the possible links existing between altered gap-junctional intercellular communication capacity (or the altered expression of their constitutive proteins, the connexins) and the stages of cancer progression in various cancer models. Then, we analyse particular aspects of the disturbance of connexin-mediated communication in cancer such as the cytoplasmic localization of connexins, the lack of heterologous communication between cancer cells and normal cells, the role of connexin gene mutations in cancer. In a separate part of the review, we also analyse the disturbance of gap-junctional intercellular communication during the late stages of cancer (invasion and metastasis processes).

Functional Domain Mapping and Selective Trans-dominant Effects Exhibited by Cx26 Disease-causing Mutations

Mutations in Cx26 are a major cause of autosomal dominant and recessive forms of sensorineural deafness. Some mutations in Cx26 are associated not only with deafness but also with skin disease. We examined the subcellular localization and function of two green fluorescent protein (GFP)-tagged Cx26 point mutants that exhibit both phenotypes, G59A-GFP and D66H-GFP. D66H-GFP was retained within the brefeldin A-insensitive trans-Golgi network, whereas a population of G59A-GFP was transported to the cell surface. Neither G59A nor D66H formed gap junctions that were permeable to small fluorescent dyes, suggesting they are loss-of-function mutations. When co-expressed with wild-type Cx26, both G59A and D66H exerted dominant-negative effects on Cx26 function. G59A also exerted a trans-dominant negative effect on co-expressed wild type Cx32 and Cx43, whereas D66H exerted a trans-dominant negative effect on Cx43 but not Cx32. We propose that the severity of the skin disease is dependent on the specific nature of the Cx26 mutation and the trans-dominant selectivity of the Cx26 mutants on co-expressed connexins. Additional systematic mutations at residue D66, in which the overall charge of this motif was altered, suggested that the first extracellular loop is critical for Cx26 transport to the cell surface as well as function of the resulting gap junction channels.

Role of paracellular junction complexes in baculovirus-mediated gene transfer to nondividing rat hepatocytes

Gene delivery to differentiated hepatocytes is notoriously difficult. Hepatocytes plated on collagen-coated dishes and maintained in dimethyl sulfoxide (DMSO)-supplemented medium acquire paracellular junctions, arrange themselves in multicellular islands and are an excellent in vitro model for studying liver function. Baculovirus-mediated gene delivery to hepatocytes in this culture system is restricted to peripheral cells of the islands. However, this limitation can be overcome by transient calcium depletion of the cells prior to and during baculovirus infection. Examination of the mechanism underlying this process revealed that calcium depletion was accompanied by a transient loss of intercellular contacts and paracellular junction complex integrity, increased distance between adjoining cells, and internalization of the tight junction protein, zona occludens ZO-1. Internalization of ZO-1 was accompanied by baculovirus infection of internal cells of hepatocyte islands. When calcium levels were restored, paracellular junction complex integrity returned to normal by 12 h. No permanent alterations in hepatocyte ultrastructure and albumin mRNA, and protein expression were caused by this gene transfer method. Loss in paracellular junction complex integrity exposes the basolateral (sinusoidal) surface of hepatocytes resulting in homogeneous baculovirus-mediated gene delivery to approximately 75% of the cells in long-term DMSO culture. We conclude that the use of recombinant baculovirus as a vector in combination with transient calcium depletion is a highly efficient method for delivering exogenous genes to hepatocytes without loss of hepatic differentiation.

Expression of E-cadherin and other paracellular junction genes is decreased in iron-loaded hepatocytes

Iron overload in the liver may occur in the clinical conditions hemochromatosis and transfusion-dependent thalassemia or by long-term consumption of large amounts of dietary iron. As iron concentrations increase in the liver, cirrhosis develops, and subsequently the normal architecture of the liver deteriorates. The underlying mechanisms whereby iron loading of hepatocytes leads to the pathology of the liver are not understood. Similarly, a direct relationship between the expression levels of paracellular junction genes and altered hepatocellular physiology has been reported; however, no relationship has been identified between iron loading and the expression of paracellular junction genes. Here, we report that the expression of numerous paracellular junction genes was decreased in iron-loaded hepatocytes, leading to increased cellular permeability, increased baculovirus-mediated gene transfer, and decreased gap junction communication. Iron loading of hepatocytes resulted in decreased E-cadherin promoter activity and subsequently decreased E-cadherin mRNA and protein expression. The data presented in this study describe a clear relationship between iron overload and decreased expression of paracellular junction genes in hepatic cells of rat and human origin.

Spatial relation between major histocompatibility complex-restricted antigen receptor-bearing thymocytes and subtypes of thymic epithelial cells

R73, a monoclonal antibody that recognizes a rat T cell surface antigen of TCR(alphabeta), was used to identify thymocytes that express major histocompatibility complex-restricted antigen receptors, and to define the spatial relation between these receptor-bearing thymocytes and individual thymic cortical and medullary epithelial subtypes by ultrastructural immunohistochemistry. We show that in both the cortex and medulla 1) the thymocytes that reacted with R73 antibodies exhibited three staining patterns: cytoplasmic-only staining, simultaneous cytoplasmic and surface membrane staining, and surface membrane-only staining; 2) the subcapsular/perivascular epithelial cells (ECs) were usually associated with thymocytes expressing perinuclear staining only; and 3) the surface membrane areas of thymocytes that expressed antigen receptors made contact with pale and intermediate ECs, but not with dark cells. These results suggest that thymic selection of major histocompatibility complex restriction and/or tolerance may occur by interaction of the receptors on maturing thymocytes with major histocompatibility complex antigens on thymic ECs in general, and on pale and intermediate subtypes in particular.

Transient disruption of intercellular junctions enables baculovirus entry into nondividing hepatocytes

Baculovirus infection has extended the capabilities for transfection of exogenous genes into a variety of mammalian cell types. Because rat hepatocytes plated on collagen-coated dishes and maintained in dimethyl sulfoxide (DMSO)-supplemented chemically defined medium are an excellent model system for studying liver function in vitro, we investigated the ability of baculoviruses to infect and deliver exogenous genes to cells in this culture system. Efficient delivery to hepatocytes in short-term culture becomes restricted to peripheral cells, or "edge" cells, as the hepatocytes acquire intercellular junctions and form islands with time in culture. This barrier to baculovirus entry can be overcome, and the percentage of internal cells within the hepatocyte islands that are infected with the baculovirus can be increased more than 100-fold, when cells are subjected to transient calcium depletion before and during infection. These findings suggest that at least in some cell types, such as hepatocytes, baculovirus entry may require contact with the basolateral surface. We conclude from this study that recombinant baculovirus infection following transient depletion of extracellular calcium results in delivery of exogenous genes to at least 75% of hepatocytes in long-term DMSO culture, thereby making it possible for the first time to carry out gain-of-function and loss-of-function studies in this cell system.

The origin of annular junctions: a mechanism of gap junction internalization

Gap junctional intercellular communication is established when connexin proteins oligomerize into connexon hemichannels, which then pair at the cell surface with connexons from neighboring cells to form functional gap junction channels. Gap junction channels routinely cluster into gap junction plaques, which can exhibit dynamic characteristics while under the frequent processes of formation and removal from the cell surface. We have three lines of evidence to suggest that one mechanism of gap junction removal occurs when one of two contacting cells internalizes the gap junction contribution from both cells. First, in coculture experiments, green fluorescent protein-tagged connexin43 (Cx43-GFP) expressed in normal rat kidney (NRK) cells can be internalized into contacting cells that do not express Cx43-GFP, and the incidences of identifying these internalized structures increase in the presence of lysosomal inhibitors. Secondly, time-lapse imaging of live NRK cells revealed that large areas of gap junction plaques containing Cx43-GFP were internalized as vesicular-like structures into one of two adjacent cells. Finally, when live NRK cells that express endogenous Cx43 were microinjected with anti-Cx43 antibodies, antibody-tagged gap junctions were visualized in cells that contacted the microinjected cell within 3–6.5 hours. Together our results strongly suggest that one mechanism of gap junction removal from the cell surface involves a unique process in which the entire gap junction or a fragment of it is internalized into one of the two contacting cells as an annular junction.

Analysis of gap junction formation in rat hepatocytes by intravenous injection of an anti-connexin32 monoclonal antibody (HAM8)

HAM8 monoclonal antibody was used to examine the mechanism of connexin32 (Cx32) formation in the rat model in vivo by immunofluorescence and immunoelectron microscopy. After a single intravenous injection of HAM8 IgG, a number of HAM8 signals were observed at the sinusoidal face, between adjacent hepatocytes as well as in the cytoplasm stained with only 2nd fluorescent antibody. Moreover, the in vivo localization of the HAM8 antigen appeared to change with time. At 5 min after the antibody injection, Cx32 signals from liver sections were clearly detected at the sinusoidal face. Fifteen minutes later, numerous linear and dotted fluorescent signals were observed between hepatocytes; in addition, much punctate staining was found at the sinusoidal face and in hepatocytes. These findings were identified by immunoelectron microscopy. Interestingly, 1 hr later, much punctate staining considered to be similar to those seen in the normal rat liver tissues was observed between adjacent hepatocytes, suggesting that a great deal of Cx32 combined with HAM8 have been assembled into identifiable gap junction plaques. Five hours later, intercellular and intracellular Cx32 signals were infrequently detected. When staining was performed with HAM8 and 2nd antibody, however, numerous Cx32 signals were again observed between neighboring hepatocytes, as punctate staining appearing in a pattern approximately the same as that seen in the normal liver tissue. Based on these results, we assumed that a precursor gap was present during Cx32 formation, and discussed the pathways of Cx32 formation and the degradation of Cx32 as well as that of HAM8.

Mutations of connexin32 in Charcot-Marie-Tooth disease type X interfere with cell-to-cell communication but not cell proliferation and myelin-specific gene expression

Connexin32 (Cx32) is a gap junction protein and its mutations are responsible for X-linked Charcot-Marie-Tooth disease. We examined the functional abnormality of C6 glioma cells transfected with mutant (C53S and P172R) Cx32 genes. Nontransfected C6 did not express Cx32. Northern and Western blot analyses showed Cx32 mRNA and protein in cells with the wild-type gene as well as with the mutant Cx32 genes. An immunocytochemical study of cells with the wild-type gene showed the immunoreactive spots in the cell membrane. In cells with C53S or P172R mutant gene, however, the immunoreactivity was found in the cytoplasm. The scrape-loading method produced effective dye transfer in cells with the wild-type gene but not in those with mutant genes. A cell proliferation assay showed no differences in nontransfected cells, cells with the wild-type gene and those with the mutant genes. Messenger RNA expression for proteolipid protein did not change. These findings suggest that Cx32 gene mutation results in loss of cell-to-cell communication because of failure to incorporate Cx32 protein in the cell membrane. The mutations do not, however, interfere with cell proliferation or myelin-specific gene expression, at least myelin proteolipid protein expression in C6 glioma cells.

Analysis of fetal rat liver using monoclonal antibodies

BACKGROUND

In previous studies, we developed several monoclonal antibodies (mAbs) against fetal and adult rat liver to analyze the hemopoietic microenvironment of the fetal liver during the gestational period. In this study, we have developed two new mAbs against fetal rat liver cells and have examined the characteristics at various gestational ages of fetal liver and of adult liver.

METHODS

The characteristics of these monoclonal antibodies were demonstrated by examining several tissues using immunohistochemical staining and flow cytofluorometry.

RESULTS

Monoclonal antibodies HAM10 and HAM11 were developed against fetal rat liver cells. These reacted with the cytoplasm of fetal and adult hepatocytes. HAM10 antigen expression was strong at approximately day 18 of gestation in the active period of hemopoiesis in fetal rat liver but was much lower in adult liver. HAM10 antigen expression also increased in liver after partial hepatectomy and was reduced abruptly to a normal level thereafter. HAM11 antigen expression in fetal liver was weaker than that of HAM10 antigen expression. The degree of HAM11 antigen expression increased as gestation proceeded, reaching a maximum in adult liver.

CONCLUSIONS

Both HAM10 and HAM11 antigens may play a role in the morphogenesis of hepatocytes and in the hemopoietic microenvironment for hemopoietic cells. Moreover, HAM10 antigen is may play a role in hepatocyte proliferation in the fetal liver, whereas HAM11 antigen may contribute to the maturation of fetal- to the adult-type hepatocytes.

Developmental expression of connexins in the chick embryo myocardium and other tissues

BACKGROUND

Connexins are cell surface proteins that form specialized regions of cell-cell communication called gap junctions. These allow impulse conduction in in voluntary muscle tissue such as the heart, but also allow the formation of communities of like cells during development of organs.

METHODS

We used an antipeptide antibody to connexin 43 in immunolocalization studies and an anti-peptide antibody to an external loop domain common to most connexins in Western blotting of total heart protein to measure the accumulation of connexins in the heart as it develops from 33 hours to 21 days (hatching), and in the adult.

RESULTS

Immunolocalization revealed that connexin 43 is widely distributed in the earliest organ rudiments. It is especially prominent in the neural tube and its derivatives, in the lens and nasal placodes, in the foregut and its derivatives, in the somites, in the mesonephric tubules, and in the heart and major arteries. Heart tissue staining grew more intense with development through day 8. However, at day 11 and day 15, and in the adult, heart staining diminished. Endocardium and valve tissue did not stain. Western blotting of heart homogenates with the antibody directed against the external loop domain peptide showed 26, 32, 43, 45, and 56 kilodalton connexins, which changed in relative abundance, displaying unique patterns during development.

CONCLUSIONS

Our results show patterns of connexin immunolocalization in early germ layers and organ rudiments that are similar to those known in the mouse, but with certain differences. Our results show a distinctive pattern of multiple connexin gene expression in the developing heart from days 2-21.

Membrane antigen expression of syngeneically but heterotopically transplanted hepatocytes in rats

The expression of membrane antigens on rat hepatocytes transplanted syngeneically and heterotopically was analyzed immunohistochemically using monoclonal antibodies against rat hepatocytes. Isolated adult and fetal hepatocytes were able to survive in the spleen, salivary gland, thymus, or subcapsular region of the kidney for various periods after transplantation. Fairly clear expression of HAM2, 4, and 8 antigens was observed on hepatocytes transplanted into syngeneic spleen, suggesting that the cells might be functionally equivalent to hepatocytes in situ. HAM4 antigen was localized specifically on the newly formed bile-canalicular faces of hepatocytes. The expression of HAM2 (MHC class I) antigen on the transplanted hepatocytes appeared much stronger on the side facing lymphoid tissues, than on the other faces, suggesting that some immunological reactions may take place between hepatocytes and lymphoid tissue. HAM8 antigen, which is localized on gap junctions between neighboring hepatocytes in rat liver, was also recognized between transplanted hepatocytes. In salivary glands where hepatocytes were transplanted, bile-canaliculus-like structures were observed not only between neighboring hepatocytes but also between hepatocytes and salivary acinar cells, suggesting good interaction between the two different epithelial cell types. Hepatocytes transplanted into thymus appeared viable, but most showed fatty degeneration. Some healthy hepatocytes survived in the interlobular connective tissue and the thymic cortical tissue. When fetal hepatocytes were transplanted heterotopically, they formed a mass consisting of hepatocytes and bile duct-like structures 7 wk after transplantation. The inoculated hepatocytes possessed HAM4 antigen, which was not recognized on fetal hepatocytes at day 14 of gestation.(ABSTRACT TRUNCATED AT 250 WORDS)