Functional demonstration of connexin-protein binding using surface plasmon resonance

Surface plasmon resonance (SPR) allows examination of protein-protein interactions in real time, from which both binding affinities and kinetics can be directly determined. We have used the SPR technique to search for proteins in heart tissue that would be candidate binding partners for the cardiac gap junction protein, connexin43 (Cx43). Heart lysate showed a strong, pH-dependent binding to the carboxyl terminus (CT) of Cx43 (amino acids 254-382) covalently linked to an SPR cuvette. Binding was inhibited by the presence of v-src transfected 3T3 cell lysate, suggesting that binding partners in these two lysates may compete for overlapping epitopes on Cx43CT. The combined application of proteomic and functional studies is expected to identify which proteins within heart tissue interact with Cx43 and what roles they may play in gap junction function.

Use of antibodies in the analysis of connexin 43 turnover and phosphorylation

A series of antipeptide antibodies designed to recognize specific sequences of the gap junction protein connexin 43 (Cx43) were developed and characterized immunochemically and immunohistologically. These antibodies bound to gap junctions and, on Western blots, to 43-kDa (often resolved as a doublet) and 41-kDa proteins in samples from heart, leptomeningeal cells, and brain. Relatively little of the 41-kDa protein was detectable in heart homogenates. Cultured rat leptomeningeal cells expressed high levels of the gap junction protein Cx43 and were used to analyze its turnover and phosphorylation. Pulse-chase experiments in leptomeningeal cells with [(35)S]methionine indicated that the 41-kDa form of connexin 43 was the first immunoprecipitable translation product. Radiolabel subsequently appeared in the lower band of the doublet at 43 kDa, followed by a shift into the higher band and turnover of the protein with a t(1/2) of 2.7 h. Pulse-chase labeling with [(32)P]P(i) indicated that phosphorylation of connexin 43 was limited to the 43-kDa protein, with a t(1/2) of 1.7 h. Treatment with alkaline phosphatase shifted the apparent molecular mass of the 43-kDa protein doublet such that it comigrated with the 41-kDa form. Hence, the 43-kDa protein observed on Western blots of both leptomeningeal cells and heart arises by phosphorylation of the 41 kDa precursor. Phosphorylation of serine residues accounts for most, if not all, of Cx43 phosphorylation in this system.

Induction of tight junctions in human connexin 32 (hCx32)-transfected mouse hepatocytes: connexin 32 interacts with occludin

Small gap junction plaques are associated with tight junction strands in some cell types including hepatocytes and it is thought that they may be closely related to tight junctions and the establishment of cell polarity. In order to examine roles of gap junctions in regulating expression and structure of tight junctions, we transfected human Cx32 cDNA into immortalized mouse hepatocytes (CHST8 cells) which lack endogenous Cx32 and Cx26. Immunocytochemistry revealed that endogenous integral tight junction protein occludin was strongly localized and was colocalized with Cx32 at cell borders in transfectants, whereas neither was detected in parental cells. In Northern blots, mRNAs encoding occludin and the other integral tight junction proteins, claudin-1 and -2, were induced in the transfectants compared to parental cells. In Western blots, occludin protein was increased in the transfectants compared to parental cells, and binding of occludin to Cx32 protein was demonstrated by immunoprecipitation. In freeze fracture of the transfectants, tight junction strands were more numerous and complex compared to parental cells, and small gap junction plaques appeared within induced tight junction strands. Nevertheless, no change in barrier function of tight junctions was observed. These results indicate that in hepatocytes, gap junction, and tight junction expression are closely coordinated, and that Cx32 may play a role in regulating occludin expression.

TPA induced expression and function of human connexin 26 by post-translational mechanisms in stably transfected neuroblastoma cells

Connexin 26 (Cx26) has been proposed to be a tumor suppressor gene and its expression may modulate development, cell growth and differentiation in various tissues, including the brain. 12-O-tetradecanoylphorbol-13-acetate (TPA) may serve as either tumor promoter (in mammary gland amd skin) or as a differentiating agent (in neuroblastoma and leukemic cells) and may also modulate expression, function and phosphorylation of gap junctions. In this study, to determine the effects of TPA on Cx26 expression and its function in neuroblastoma, we transfected N2A mouse neuroblastoma cells (which are gap junction deficient) with the coding region of human Cx26 gene (which lacks TPA response elements) and examined the changes of expression and function of Cx26 following 10 nM TPA treatment. Individual clones of transfectants stably expressed distinct levels of exogenous Cx26 as judged by Northern and Western blots, immunocytochemistry and electrophysiological recordings. Cx26 channels displayed unitary conductances of about 140-155 pS. Increase of Cx26 expression following TPA treatment was markedly observed using immunocytochemistry and Western blots of membrane fractions although it was not detected in Northern or Western blots of whole cells. This increase in Cx26 expression in the plasma membrane was accompanied by an increase of function as evidenced in measurements of junctional conductance. These results suggest that induction of exogenous Cx26 in neuroblastoma cells by TPA treatment is controlled by post-translational mechanisms.

Deficient assembly and function of gap junctions in Trf1, a trafficking mutant of the human liver-derived cell line HuH-7

The Trf1 cell line, selected from the human hepatoma cell line HuH-7, manifests altered trafficking of various plasma membrane proteins. In particular, there is a striking loss of State 2 asialoglycoprotein receptors. This cell line is shown here to also manifest defects in function and assembly of gap junctions comprising connexin43 (Cx43). No alteration of Cx43 expression or phosphorylation was apparent. Nevertheless, immunostaining of Cx43 revealed that fewer and smaller gap junctions were present at appositional membrane areas in Trf1 cells as compared with parental HuH-7. This correlated with a significant attenuation in gap junction-mediated communication between Trf1 cells as demonstrated by markedly decreased dye transfer and their reduced ability to propagate mechanically evoked Ca(2+) waves. Isoelectric focusing (IEF) of Cx43 in HuH-7 cells indicated that the pIs of this protein were significantly lower than that predicted from its amino acid sequence; no differences in pI were evident in Cx43 from Trf1 cells and the HuH-7 cell line. The effects of the Trf1 mutation on assembly and function of gap junctions indicate that this mutation influences trafficking of Cx43. Connexins differ in several respects from other membrane proteins thus far analyzed in Trf1 mutants: gap junctions localize exclusively to the lateral cell surface; they are not glycoproteins; and they do not play a role in endocytic pathways. The disruption of trafficking of Cx43 by this mutation suggests that the Trf1 phenotype is a defect at a common point along the trafficking pathway of cell-surface proteins, irrespective of their ultimate destination on the cell surface or their glycosylation profile.

Association of cell and substrate adhesion molecules with connexin43 during intramembranous bone formation

Prior studies in our laboratory have demonstrated an association of specific gap junction proteins with intramembranous bone formation in the avian mandible. The purpose of the present study was to extend these observations by determining if there was a relationship between the expression of one of the gap junction proteins examined previously (connexin43) and the expression of specific cell adhesion (CAM) and/or substrate adhesion (SAM) molecules [i.e. NCAM, A-CAM (N-cadherin) and tenascin (tenascin-C)] that have previously been shown to be associated with bone formation. Immunohistochemical localization of connexin43, tenascin, NCAM and N-cadherin was performed on serial sections of mandibles of chick embryos from 6 to 12 days of incubation. Analysis of adjacent serial sections revealed that the NCAM and tenascin immunostaining that appeared initially on the lateral aspect of Meckel's cartilage preceded the overt expression of trabecular bone. At subsequent stages, NCAM and tenascin staining gradually overlapped the region of connexin43 expression. In contrast, the expression of N-cadherin was found to colocalize with that of connexin43 from the first appearance of connexin43 expression. Most significantly, although the domains of NCAM and tenascin expression were initially separate from that of connexin43, bone formation originated only in the region where these domains intersected. These findings suggest that, of the CAMs and SAMs examined, N-cadherin appears to be associated with the establishment of cell contacts responsible for the presence and/or maintenance of connexin43-mediated gap junctional communication, while tenascin and NCAM appear to be associated, in a more specific manner, with processes that accompany the overt expression of the osteogenic phenotype.

Immunorecognition, ultrastructure and phosphorylation status of astrocytic gap junctions and connexin43 in rat brain after cerebral focal ischaemia

Gap junctions between astrocytes support a functional syncytium that is thought to play an important role in neural homeostasis. In order to investigate regulation of this syncytium and of connexin43 (Cx43), a principal astrocytic gap junction protein, we determined the sequelae of gap junction and Cx43 disposition in a rat cerebral focal ischaemia model with various ischaemia/reperfusion times using sequence-specific anti-Cx43 antibodies (designated 13-8300, 18A, 16A and 71-0700) that exhibit differential recognition of Cx43, perhaps reflecting functional aspects of gap junctions. Antibody 13-8300 specifically detects only an unphosphorylated form of Cx43 in both Western blots and tissue sections. In hypothalamus after brief (15 min) ischaemic injury, Cx43 at intact gap junctions undergoes dephosphorylation, accompanied by reduced epitope recognition by antibodies 16A and 71-0700. Tissue examined 24 h after reperfusion showed that these effects were reversible. Astrocytic gap junction internalization occurring 1 h after ischaemia was accompanied by decreased immunodetection with 13-8300. At this time, gap junctions were absent in the ischaemic core, coinciding with a loss of Cx43 recognition with 18A and 13-8300, but elevated labelling of internalized Cx43 with 16A and 71-0700. Unphosphorylated Cx43 persisted at intact gap junctions confined to a thin corridor at the ischaemic penumbra which contained presumptive apoptotic cell profiles. Similar results were obtained in ischaemic striatum and cerebral cortex, though with a delayed time course that depended on the severity of the ischaemic insult. These results demonstrate that astrocytic Cx43 epitope masking, dephosphorylation and cellular redistribution occur after ischaemic brain injury, proceed as a temporally and spatially ordered sequence of events and culminate in differential patterns of Cx43 modification and sequestration at the lesion centre and periphery. These observations suggest an attempt by astrocytes in the vicinity of injury to remodel the junctional syncytium according to altered tissue homeostatic requirements.

Gap junction disappearance in astrocytes and leptomeningeal cells as a consequence of protozoan infection

Trypanosoma cruzi and Toxoplasma gondii are protozoan parasites capable of causing infections of the nervous system. In order to determine effects of infection by these organisms on intercellular communication in the brain, dye coupling and connexin abundance and distribution were examined in leptomeningeal cells and astrocytes infected with T. cruzi or T. gondii. For both cell types infected with either type of protozoan parasite, intercellular diffusion of intracellularly injected Lucifer Yellow was dramatically reduced. Immunocytochemistry with antibodies specific for connexin43 (in astrocytes) or both connexin43 and connexin26 (for leptomeningeal cells) demonstrated that punctate gap junctional staining was much reduced in infected cells, although uninfected neighbors could display normal connexin abundance and distribution. Western blot analyses revealed that connexin43 abundance in both cell types infected with either parasite was similar to that in uninfected cells. Phosphorylation state of connexin43 (inferred from electrophoretic mobility of connexin43 isoforms) was not significantly affected by the infection process. Immunocytochemistry of whole brains from animals acutely infected with either parasite also showed a marked reduction in connexin43 expression. We conclude that infection of both types of brain cells with either protozoan parasite results in a loss of intercellular communication and organized gap junction plaques without affecting expression levels or posttranslational processing of gap junction proteins. Presumably, these changes in gap junction distribution result from altered targeting of the junctional protein to the plasma membrane, and/or from changes in assembly of subunits into functional channels.

Selective monoclonal antibody recognition and cellular localization of an unphosphorylated form of connexin43

A sequence-specific monoclonal antibody directed against the gap junction protein connexin43 (Cx43) is shown here to be specific for the unphosphorylated form of this protein. In tissues and cultured cells containing different phosphorylated and unphosphorylated forms of Cx43, the antibody detected only the latter as shown by Western blotting of native and alkaline phosphatase-treated samples. Immunohistochemically, this monoclonal antibody did not recognize gap junctions in the vast majority of cultured cardiac myocytes, where nearly all detectable Cx43 is phosphorylated. In contrast, it was able to detect some intracellular Cx43 in tracheal smooth muscle cells and an epithelial cell line (Cl-9 cells), producing patterns of labeling consistent with those seen using a polyclonal antibody that recognizes both phosphorylated and unphosphorylated forms of Cx43. Immunostaining of gap junctions in the cultured cells indicates that both phosphorylated and unphosphorylated Cx43 are present in some assembled gap junctions, suggesting that assembled junctions do not contain exclusively the phosphorylated form of the protein. Annular gap junctions, believed to form as part of the pathway for internalization and degradation of gap junctions, were only occasionally and sparsely labeled by the monoclonal antibody, indicating that complete protein dephosphorylation is not required for uptake and degradation of gap junctions. Furthermore, the ability of this antibody to recognize only unphosphorylated Cx43, and not any of the phosphorylated forms present in the tissues and cell types examined, suggests that a unique phosphorylation site, perhaps present in the epitope recognized by this antibody, must be phosphorylated prior to phosphorylation of Cx43 at other sites.

Phosphorylation of connexin43 and the regulation of neonatal rat cardiac myocyte gap junctions

The functional state of gap junctions and the state of phosphorylation of connexin43 (Cx43), the major gap junction protein in rat heart, were evaluated in primary cultures of neonatal rat cardiocytes. Functional coupling was greatly reduced after treatment with staurosporine (ST), a protein kinase inhibitor. The ST-induced reduction in cell coupling was reversed by activation of protein kinase C (PKC) with 12-O-tetradecanoylphorbol 13-acetate (TPA). The cellular distribution of Cx43, as detected by immunofluorescence, was not grossly affected by either ST alone or ST plus TPA. Although immunoblot analysis did not detect significant changes in the relative amounts of the unphosphorylated and individual phosphorylated forms of Cx43 after each treatment, the level of 32P-incorporation into Cx43 of radiolabeled cells was significantly affected. Consistent with their known properties, treatment with ST reduced, and combined treatment with TPA and ST increased, the level of 32P-incorporation into Cx43. Two-dimensional tryptic phosphopeptide maps of 32P-labeled Cx43 indicated that a distinct subset of the phosphopeptides that are present under basal conditions were affected by ST or ST/TPA treatments, with TPA-induced phosphorylation occurring at the ST-sensitive sites. However, the ST/TPA-sensitive tryptic phosphopeptides did not comigrate with others that were derived from in vitro phosphorylation by PKC of a recombinant C-terminal Cx43 peptide (Cx43[243-382]). Although a PKC-dependent mechanism appears to be involved in the regulation of functional coupling between neonatal rat cardiocytes, PKC itself may not be the final mediator of Cx43 phosphorylation.

Connexin43 and astrocytic gap junctions in the rat spinal cord after acute compression injury

To examine the possible role of interastrocytic gap junctions in the maintenance of tissue homeostasis after spinal cord damage, we initiated studies of the astrocytic gap junctional protein connexin43 (Cx43) in relation to temporal and spatial parameters of neuronal loss, reactive gliosis, and white matter survival in a rat model of traumatic spinal cord injury (SCI). Cx43 immunolocalization in normal and compression-injured spinal cord was compared by using two different sequence-specific anti-Cx43 antibodies that have previously exhibited different immunorecognition properties at lesion sites in brain. At 1- and 3-day survival times, gray matter areas with mild to moderate neuronal depletion exhibited a loss of immunolabeling with one of the two antibodies. At the lesion epicenter, these areas consisted of a zone that separated normal staining distal to the lesion from intensified labeling seen with both antibodies immediately adjacent to the lesion. Loss of immunoreactivity with only one of the two antibodies suggested masking of the corresponding Cx43 epitope. By 7 days post-SCI, Cx43 labeling was absent with both antibodies in all regions extending up to 1 mm from the lesion site. Reactive astrocytes displaying glial fibrillary acidic protein (GFAP) appeared by 1 day and were prominent by 3 days post-SCI. Their distribution in white and gray matter corresponded closely to that of Cx43 staining at 1 day, but less so at 3 days when GFAP-positive profiles were present at sites where Cx43 labeling was absent. By 7 days post-SCI, Cx43 again co-localized with GFAP-positive cells in the surviving subpial rim, and with astrocytic processes on radially oriented vascular profiles investing the central borders of the lesion. The results indicate that alterations in Cx43 cellular localization and Cx43 molecular modifications reflected by epitope masking, which were previously correlated with gap junction remodeling following excitotoxin-induced lesions in brain, are not responses limited to exogenously applied excitotoxins; they also occur in damaged spinal cord and are evoked by endogenous mechanisms after traumatic SCI. The GFAP/Cx43 co-localization results suggest that during their transformation to a reactive state, spinal cord astrocytes undergo a transitional phase marked by altered Cx43 localization or expression.

Oligodendrocytes express gap junction proteins connexin32 and connexin45

Oligodendrocytes, the myelin-forming glia of brain, are connected by gap junctions in situ and in culture. Cultured oligodendrocytes from adult bovine and porcine brains were studied using immunocytochemical, molecular, and electrophysiological techniques in order to characterize the gap junction types. The expression of connexin32 was substantiated by the detection of low, but significant, signals using connexin-specific probes in Northern and Western blot analyses. Connexin43, which comprises gap junctions in astrocytes, was not detectable in pure oligodendrocytic cultures; mRNAs of connexin40 and connexin37 and connexin26 were also not detected. By means of two specific antibodies directed to the recently cloned connexin45 and by RT-PCR we were able to identify this connexin as a second oligodendrocytic gap junction protein. Whole cell voltage clamp recording provided evidence for electrical coupling between pairs of cultured oligodendrocytes (mean junctional conductance 3.9 nS, n = 38 pairs) and intracellular Lucifer Yellow injection indicated that oligodendrocytes were usually only weakly dye coupled, with spread generally being restricted to nearest neighbors. Unitary conductances ranged from > 20 to < 150 pS with modes of distribution at about 100 to 120pS and 40 to 20 pS, respectively. These unitary conductances are consistent with the channel events expected for connexin32 and connexin45. The low degree of functional coupling between oligodendrocytes in vitro corresponds with the low levels of connexin32 and connexin45 messenger RNAs and protein expression.

Expression patterns of connexin43 protein during facial development in the chick embryo: associates with outgrowth, attachment, and closure of the midfacial primordia

BACKGROUND

In a prior report, evidence was presented for the presence of gap junction proteins [connexin32 and connexin43 (Cx43)] in embryonic facial primordia. The purpose of the present study was, first, to examine in detail the patterns of distribution of Cx43 protein in embryonic chick facial primordia and, second, to consider the possible roles played by this protein during midfacial development.

METHODS

Chick embryo heads were serially sectioned and processed for immunofluorescent localization of Cx43. The developmental stages examined encompassed the period of formation, enlargement, and union of the facial primordia. Western blot analysis of the facial primordia was also performed.

RESULTS

Analysis of serial sections revealed the presence of signal in both epithelium and mesenchyme at sites of attachment in each of the midfacial primordia (i.e., the medial nasal, lateral nasal, and maxillary processes). Furthermore, although signal was concentrated in mesenchyme in the distal tips of the primordia at sites of attachment, immunoreactivity was absent, sparse, or less intense outside the areas of attachment. In some cases (i.e., the maxillary process), immunoreactive signal in mesenchyme did not appear in the distal tip until the primordia approximated each other or contact of the primordia was initiated. Most significantly, signal was also found between the facial primordia in nonprimordial epithelium and mesenchyme at sites where the primordia were joined.

CONCLUSIONS

These data suggest that the expression of Cx43 protein is spatially and temporally regulated in the facial primordia and that the patterns of expression that were observed are significant to the cascade of events that ultimately lead to the attachment and union of the primordia that form the midface.

Evidence for the co-localization of another connexin with connexin-43 at astrocytic gap junctions in rat brain

Gap junctions between astrocytes as well as between astrocytes and oligodendrocytes in rat brain were immunohistochemically labelled with a monoclonal and an affinity-purified polyclonal antibody generated against connexin-26. By light microscopy, the immunolabelling patterns obtained were, with a few exceptions, remarkably similar to previously described distribution patterns of the gap junctional protein connexin-43, which is expressed by astrocytes and is localized at astrocytic gap junctions. By electron microscopy, immunoreactivity with these two anti-connexin-26 antibodies was restricted to astrocytes; inter-astrocytic gap junctional membranes were symmetrically labelled, heterologous oligo-astrocytic junctional membranes were asymmetrically labelled only on the astrocyte side and oligo-oligodendrocyte junctions were unlabelled. Two additional anti-connexin-26 antibodies that were found to produce punctate labelling in leptomeninges and liver failed to do so in brain parenchyma, consistent with reports indicating the absence of authentic connexin-26 in this tissue. Antibodies that labelled astrocytic gap junctions exhibited no cross-reaction with connexin-43 or connexin-32, as demonstrated by western blotting, but recognized liver connexin-26 as well as several brain proteins, including an approximately 32000 mol. wt protein that did not correspond to connexin-32 and a 26000 mol. wt protein that co-migrated with liver connexin-26. These results suggest that connexin-26, or more likely a protein having sequence homology with connexin-26, is targeted to astrocytic gap junctions and raise the possibility of the existence of connexins that may be co-expressed with connexin-43 in most, but perhaps not all, astrocytes.

Connexin32 in oligodendrocytes and association with myelinated fibers in mouse and rat brain

The distribution and cellular localization of connexin32 (Cx32) in the brain and spinal cord of the mouse and rat was investigated by light microscope (LM) and electron microscope (EM) immunohistochemistry by using several different antibodies against Cx32. By double immunofluorescence staining for Cx32 and either the oligodendrocyte markers cyclic nucleotide phosphodiesterase (CNPase) or Rip, Cx32 was consistently found in oligodendrocyte cell bodies and proximal processes. Cx32 immunoreactivity was also clearly visualized along CNPase- and Rip-positive myelinated fibers. Both immunopositive cells and fibers were heterogeneously distributed and were often more intensely labeled when dispersed in or associated with regions of gray matter than when concentrated in major white matter tracts. Labeling of myelin sheaths along fibers was restricted to subpopulations of myelinated axons. In the cerebellar cortex, for example, it was selectively localized to sheaths around Purkinje cell axons. Punctate staining, distinct from that corresponding to cells or fibers, was evident in the olfactory bulb and hippocampus. By EM, oligodendrocytes exhibited cytoplasmic labeling associated with rough endoplasmic reticulum and Golgi apparatus. Their processes were intermittently stained, most intensely when surrounding myelinated fibers and occasionally in paranodal loops. Cx32-immunoreactive gap junctions with symmetric labeling (staining on both junctional membranes) were observed between oligodendrocytic somata and processes as well as between presumptive oligodendrocytic processes. Unidentifiable elements forming asymmetrically labeled gap junctions (staining only one side of junctional membranes) were less frequently encountered. Western blot analysis confirmed anti-Cx32 antibody detection of Cx32 in whole brain homogenates and an enrichment of the protein in isolated myelin fractions. These results are consistent with earlier ultrastructural studies showing the occurrence of inter-oligodendrocytic gap junctions, but indicate that these may be more prevalent than previously thought. Furthermore, the results suggest a specialized role of gap junctions composed of Cx32 along myelinated fibers belonging to subpopulations of neurons.

Connexin-43 in rat spinal cord: localization in astrocytes and identification of heterotypic astro-oligodendrocytic gap junctions

Connexin-43 in relation to gap junctions between astrocytes and between other cell types in rat spinal cord was investigated immunohistochemically. In gray matter, connexin-43 was distributed thoughout all laminae, but was more concentrated in the substantia gelatinosa and around the central canal. Ultrastructurally, immunostaining was present in the cytoplasm of, and at gap junctions between, fine astrocytic processes, most of which ensheathed neuronal elements. In white matter, connexin-43 was localized to somata of fibrous astrocytes, their glial fibrillary acidic protein-positive processes running parallel to myelinated axons, and at gap junctions between these processes. Labelling was also evident in thick radially-directed astrocytic processes displaying pockets of staining near immunopositive gap junctions. Near the cord surface, staining was present in cell bodies of subpial astrocytes and at gap junctions between their tangential processes which formed most of the glia limitans. Radially-directed processes of subpial astrocytes formed symmetrically- and asymmetrically-labelled gap junctions with each other and extended fine branches into surrounding white matter where they made contact and often formed gap junctions with oligodendrocytic processes at the outer surface of myelinated fibres. Immunopositive astrocyte processes also made heterologous gap junctions with unstained oligodendrocyte cell bodies. Ependymal cells lining the central canal exhibited apical cytoplasmic labelling, as well as symmetrically-labelled gap junctions at their apices. Ependymal cells also formed asymmetrically-labelled gap junctions at which the junctional membranes of unlabelled cells, presumed to be tanycytes, were unstained. The results indicate the expression of connexins in addition to connexin-43 at asymmetrically-labelled gap junctions between some astrocytic processes, between astrocytes and oligodendrocytes and between some ependymal cells. The presence of gap junctions between astrocyte and oligodendrocyte processes at the outer surface of myelin suggests incorporation of the latter into the extensive gap junctionally-coupled astrocytic syncytium.

Elevated connexin43 immunoreactivity at sites of amyloid plaques in Alzheimer's disease

The distribution of the astrocytic gap junctional protein, connexin43 (Cx43) was compared immunohistochemically with that of amyloid plaques in Alzheimer's Disease (AD) brain. By light microscopy, cortical areas containing numerous beta/A4 amyloid plaques exhibited increased immunostaining density for Cx43 and some plaques corresponded exactly to sites of intensified Cx43 immunoreactivity. By electron microscopy, Cx43 was localized to astrocytic gap junctions in AD brain. Increased Cx43 expression in AD may represent an attempt to maintain tissue homeostasis by augmented intercellular communication via gap junction formation between astrocytic processes that invest senile plaques, or alternatively, an aberrant induction of astrocytic Cx43 expression which may further compromise homeostasis and exacerbate pathological conditions in the microenvironment of amyloid plaques.

Induction of connexin43 and gap junctional communication in PC12 cells overexpressing the carboxy terminal region of amyloid precursor protein

Previous studies have shown that PC12 cells overexpressing beta/A4 amyloid peptide display altered morphology characterized by pronounced membrane ruffling and extensive intercellular appositions. Having observed other cell types in which these features accompany increased connexin43 (Cx43) production and gap junctional communication, we examined Cx43 in normal and beta/A4-transfected PC12 cells. Studies of two beta/A4-transfected PC12 clones revealed an induction of Cx43 expression by Western blotting, intracellular and plasma membrane-associated Cx43 in some cells of cultures processed by immunofluorescence, dye-transfer between some cells microinjected with Lucifer Yellow, and gap junctions between cells examined by EM. Normal and vector-transfected PC12 cells exhibited none of these properties. Increased immunofluorescence in some clusters of beta/A4-transfected cells was also observed with a monoclonal antibody against connexin32. The results suggest that beta/A4 amyloid peptide may cause aberrant intercellular communication and gap junction formation through induction or increased expression of connexins in cells that are not normally coupled or only poorly coupled by gap junctions.

Reversible intercellular coupling by regulated expression of a gap junction channel gene

Direct intercellular coupling through gap junction channels has been implicated in diverse processes including cellular differentiation, growth control, metabolic cooperativity and electronic coupling and natural and induced mutations in connexin genes have been described in human and experimental disease states. Genetic systems in which the extent of coupling could be reversibly regulated would provide an important approach for examining these potential functional roles, both in vitro and in vivo. Here we describe the generation and characterization of cell lines in which the extent of coupling is reversibly controlled at the transcriptional level. Plasmids encoding a tetracycline-controlled transactivator and a tetracycline-responsive connexin32 target gene were introduced in the communication-deficient SKHep1 cell line. Quantitative immunoblotting and confocal immunofluorescence microscopy with connexin32-specific antibodies demonstrated that expression of connexin32 in stable transfectants was tightly regulated by tetracycline treatment. Moreover, transfectants exhibited a highly coupled phenotype which was rapidly and reversibly converted to the communication deficient parental state after tetracycline treatment. Time constants for decay of the messenger RNA, protein and functional coupling were similar (approximately 4 hrs), implying that transcription was rate-limiting and that separate long-lived pools of connexin32 protein were absent. In contrast to other approaches in which the extent of coupling is pharmacologically regulated by altering channel gating characteristics or by generalized blockade of transcription or translation, in this system intercellular communication is regulated by directly controlling connexin gene expression.

Effects of cGMP-dependent phosphorylation on rat and human connexin43 gap junction channels

The effects of 8-bromoguanosine 3':5'-cyclic monophosphate (8Br-cGMP), a membrane-permeant activator of protein kinase G (PKG), were studied on rat and human connexin43 (Cx43), the most abundant gap junction protein in mammalian heart, which were exogenously expressed in SKHep1 cells. Under dual whole-cell voltage-clamp conditions, 8Br-cGMP decreased gap junctional conductance (gj) in rat Cx43-transfected cells by 24.0 +/- 3.7% (mean +/- SEM, n = 5), whereas gj was not affected in human Cx43-transfected cells by the same treatment. The relaxation of gj in response to steps in transjunctional voltage observed in rat Cx43 transfectants was best fitted with three exponentials. Time constants and amplitudes of the decay phases changed in the presence of 8Br-cGMP. Single rat and human Cx43 gap junction channels were resolved in the presence of halothane. Under control conditions, three single-channel conductance states (gammaj) of about 20, 40-45 and 70 pS were detected, the events of the intermediate size being most frequently observed. In the presence of 8Br-cGMP, the gammaj distribution shifted to the lower size in rat Cx43 but not in human Cx43 transfectants. Immunoblot analyses of Cx43 in subconfluent cultures of rat Cx43 or human Cx43 transfectants showed that 8Br-cGMP did not induce changes in the electrophoretic mobility of Cx43 in either species. However, the basal incorporation of [32P] into rat Cx43 was significantly altered by 8Br-cGMP, whereas this incorporation of [32P] into human Cx43 was not affected. We conclude that 8Br-cGMP modulates phosphorylation of rat Cx43 in SKHep1 cells, but not of human Cx43. This cGMP-dependent phosphorylation of rat Cx43 is associated with a decreased gj, which results from both an increase in the relative frequency of the lowest conductance state and a change in the kinetics of these channels.

Astrocytic gap junction removal, connexin43 redistribution, and epitope masking at excitatory amino acid lesion sites in rat brain

We previously reported that kainic acid (KA) lesion sites in rat brain exhibit an absence of astrocytic gap junctions at 1 week post-lesion. Loss of immunocytochemical reactivity with a sequence-specific antibody against the astrocytic gap junctional protein connexin43 (Cx43) suggested epitope masking since persistence of Cx43 was observed on Western blots. Here, we determined the fate of Cx43 at various times after thalamic KA and striatal NMDA lesions. In normal tissue and at 6 hr post-KA lesion, Cx43 immunoreactivity predominated at typical astrocytic gap junctions. Immunolabelled junctions were still seen at 3 days, with epitope masking already present, and were virtually absent by 6 days post-lesion. Gap junction remodeling was indicated by the appearance of intracellular immunostained annular profiles and uncharacteristically extensive gap junctions between symmetrically immunolabelled membranes and between labelled astrocytic and unlabelled oligodendrocytic membranes. Labelled multivesicular clusters emerged at 2 days, were numerous at 3 days and constituted the sole Cx43 sequestration site by post-lesion day 6. Ultrastructural disruption and gap junction disassembly progressed more slowly in NMDA-injected tissue where immunoreactivity persisted, albeit at markedly decreasing levels until the final survival time examined (16 days). Intense Cx43 immunolabelling was seen in filopodia of putative reactive astrocytes at the lesion periphery at 6-8 days and was associated at 16 days with an increased number of gap junctions primarily between fine astrocytic processes. These results demonstrate that massive neuronal loss alone or in conjunction with direct actions of excitotoxins on astrocytes precipitates an astrocytic reaction accompanied initially by removal of their gap junctions followed by redistribution of Cx43, and suggest that the astrocytic syncytium may undergo reorganization in a manner leading to isolation of the lesion site.

Proliferation-associated differences in the spatial and temporal expression of gap junction genes in rat liver

After a 70% partial hepatectomy (PH), the steady-state levels of Connexin (Cx)32, Cx26, and Cx43 messenger RNA (mRNA) transcripts each displayed unique patterns of temporal expression. Within 1 hour after surgical resection, increased expression of all three Cx mRNAs was observed. Subsequently, the level of Cx32 mRNA transcripts transiently decreased to a nadir at 12 hours. Comparisons of the spatial changes with previously reported hepatocyte proliferation kinetics induced by PH demonstrated that hepatocytes before S-phase "remodel" their GJs. Within 1 to 5 hours post-PH, midzonal hepatocytes exhibited diffuse membrane staining different from the normal punctate distribution. Subsequently, midzonal hepatocytes expressed colocalized punctate Cx32 and Cx26 immunostaining. Because the changes occurred in midzonal hepatocytes before 24 hours post-PH, near the peak of hepatocyte DNA synthesis, these findings indicate that Cx26 is enhanced in hepatocytes before the onset of S-phase. In contrast to the restricted expression of Cx43 in Glisson's capsule in adult liver, Cx43 protein and mRNA were enhanced specifically in proliferating bile duct and perisinusoidal cells post-PH. PH performed during continuous administration of 2-acetylaminofluorene (AAF) prevented changes in Cx32 and Cx26 staining observed in the absence of AAF. Proliferating oval cells were found to express diffuse Cx43 immunoreactivity. On day 11 post-PH and AAF, basophilic hepatocytes displayed both punctate Cx32 and Cx26 staining, whereas bile ducts and perisinusoidal cells expressed Cx43. These findings indicate that alterations in Cx32 and Cx26 expression occur rapidly in hepatocytes stimulated to proliferate and that several nonparenchymal liver cell types upregulate Cx43 expression when induced to proliferate. Differentiation of oval cells into basophilic hepatocytes resulted in their expression of Cx32 and Cx26.

In situ transblot and immunocytochemical comparisons of astrocytic connexin-43 responses to NMDA and kainic acid in rat brain

Intracerebral injection of kainic acid (KA) in rat brain was previously found to cause altered immunohistochemical recognition of connexin-43 (Cx43) epitopes (epitope masking) with different sequence-specific antibodies against this gap junction protein. We demonstrate here that similar alterations occur when nitrocellulose membranes containing protein transferred from fresh cryostat sections of KA-injected brain are probed with these antibodies (in situ transblotting), indicating that epitope masking is not a result of epitope alteration due to fixation conditions used in earlier studies. Alterations in immuno-recognition of astrocytic Cx43 subsequent to injections of NMDA were also observed and were similar to those seen with KA in some, but not all respects. The results provide further indications of Cx43 molecular modification in excitotoxin-lesioned tissue and suggest that the sequelae of reactions by astrocytes and their gap junctions in these tissues is dependent on cell-type susceptibility to excitotoxin action.

Cell-to-cell communication in osteoblastic networks: cell line-dependent hormonal regulation of gap junction function

We have characterized the distribution, expression, and hormonal regulation of gap junctions in primary cultures of rat osteoblast-like cells (ROBs), and three osteosarcoma cell lines, ROS 17/2.8, UMR-106, and SAOS-2, and a continuous osteoblastic cell line, MC3T3-E1. All cell lines we examined were functionally coupled. ROS 17/2.8 were the more strongly coupled, while ROB and MC3T3-E1 were moderately coupled and UMR-106 and SAOS-2 were weakly coupled. Exposure to parathyroid hormone (PTH) for 1 h increased functional coupling in ROB cells in a concentration-dependent manner. Furthermore, PTH(3-34), an analog of PTH with binds to the PTH receptor and thus attenuates PTH-stimulated cAMP accumulation, also attenuated PTH-stimulated functional coupling in ROB. This suggests that PTH increases functional coupling partly through a cAMP-dependent mechanism. A 1 h exposure to PTH did not affect coupling in ROS 17/2.8, UMR-106, MC3T3-E1, or SAOS-2. To examine whether connexin43 (Cx43), a specific gap junction protein, is present in functionally coupled osteoblastic cells, we characterized Cx43 distribution and expression. Indirect immunofluorescence with antibodies to Cx43 revealed that ROS 17/2.8, ROB, and to a lesser extent MC3T3-E1 and UMR-106, expressed Cx43 immunoreactivity. SAOS-2 showed little if any Cx43 immunoreactivity. Cx43 mRNA and Cx43 protein were detected by Northern blot analysis and immunoblot analysis, respectively, in all cell lines examined, including SAOS-2. Our findings suggest that acute exposure to PTH regulates gap junction coupling, in a cell-line dependent manner, in osteoblastic cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and functional studies on rat liver fat-storing cell line and freshly isolated hepatocyte coculture system

We developed and characterized a coculture system composed of a fat-storing cell clone (CFSC-2G) and freshly isolated hepatocytes that can reproduce in vitro some of the physical and functional relationships observed in vivo. Hepatocytes in the coculture are polarized, are smaller in size than hepatocytes plated on plastic, maintain a cuboidal shape, and have a tendency to form cords. Fat-storing cells, which are initially extended, retract and leave spaces that resemble liver sinusoids. Both cell types in the coculture system are functional for at least two weeks as determined by the expression of high levels of liver-specific protein mRNAs as well as by the production and secretion of liver-specific proteins into the culture medium. The hepatocytes maintain relatively high levels of asialoglycoprotein receptor on their cell surface and form functional gap junctional complexes with fat-storing cells. Hence, this coculture system retains a number of differentiated functions of hepatocytes, making it a useful model to study cell-cell interactions in culture and to analyze regulation of hepatocyte functions.

The dendritic lamellar body: a new neuronal organelle putatively associated with dendrodendritic gap junctions

It is shown in rat that antiserum alpha 12B/18 specifically labels a new lamellar organelle that is exclusively located in dendritic appendages. These dendritic lamellar bodies occur in a restricted number of brain regions, which include areas like the inferior olive, area CA1 and CA3 of the hippocampus, dentate gyrus, olfactory bulb, and cerebral cortex. In these regions the neurons with lamellar bodies form dendrodendritic gap junctions. Immunoreactivity in the inferior olive is first detected between P9 and P15, which coincides with the development of gap junctions in this nucleus. In the adult inferior olive, the density of dendritic lamellar bodies is highest in the rostral medial accessory olive, the subnucleus where electrotonic coupling is most prominent. Antiserum alpha 12B/18, thus, specifically detects a new neuronal organelle that may be related to dendrodendritic gap junctions.

Differences in the expression of connexin genes in rat hepatomas in vivo and in vitro

Gap-junctional intercellular communication (GJIC) in normal rat liver cells involves at least three different connexins (Cxs)--Cx32, Cx26, Cx43--depending on the cell type, position in the lobule, or both. Whereas rat hepatocyte primary cultures expressed Cx32 and Cx26 as observed in vivo, cell lines derived from normal rat liver (WB-F344, Clone 9, RLEC, and BRL) expressed Cx43 and to a lesser extent Cx26. Hepatoma cells propagated in vitro were either deficient in GJIC and Cx expression (7777, 8994, H4IIE-C3) or communicated via gap junctions composed of Cx43 protein (N1S1-67, 9618A). Analysis of neoplasms that resulted from injection of hepatoma cells into rat femoral muscle showed differences in Cx expression when compared with cells grown in vitro. Whereas hepatoma cells 7777 and H4IIE-C3 failed to express Cx mRNAs in culture, these cells transplanted in vivo expressed levels of Cx32 mRNA comparable to those in normal liver. However, detectable Cx32 immunostaining was observed in less than 5% of the neoplastic cells in vivo. These results indicate that Cx32 protein was posttranscriptionally downregulated in 7777 and H4IIE-C3 tumor cells. Unexpectedly, 9618A cells expressed Cx43 mRNA and protein in cell culture but expressed Cx32 mRNA in vivo. In contrast, N1S1 transplants continued to express Cx43 mRNA and protein in vivo. Unlike the punctate Cx43 staining observed in suspension cultures of N1S1 cells, diffuse intracellular Cx43 staining was observed in N1S1-derived neoplasms in vivo, although the electrophoretic pattern of Cx43 isolated from N1S1 tumors grown in vivo (43 kDa) was different from that observed in suspension cell cultures (43 and 45 kDa). Thus, the findings reported here demonstrate that Cx expression in hepatoma cells depends on the environment, whether in vivo or in vitro, in which the cells are propagated.

Immunolocalization of connexin 43 in the tooth germ of the neonatal rat

Rabbit polyclonal antibodies to amino acids 346-360 of connexin 43, the 'heart' gap junction protein, were employed to immunolocalize connexin 43 gap junctions in the neonatal rat molar tooth germ. Connexin 43 appears early in the differentiation of both ectodermally derived and ectomesenchymally derived cells of the developing tooth. Connexin 43 immunoreactivity is present in the epithelial components of the enamel organ, including the area of the proximal and distal junctional complexes of the ameloblast layer, and the stratum intermedium, stellate reticulum and outer enamel epithelium. Secretory odontoblasts and developing alveolar bone also display a pattern of connexin 43 immunostaining. Both the epithelial and ectomesenchymally-derived components of the developing tooth acquire connexin 43 channels in a manner that correlates with cell differentiation. In addition, three regions can be defined by connexin 43 immunostaining: the epithelia of the enamel organ that are derived from the oral epithelium, the odontoblast layer derived from the ectomesenchyme, and the alveolar bone. The results suggest that connexin 43 may provide the mechanism for functional compartmentalization of the tissues associated with tooth formation. Compartmentalization suggested by connexin 43 expression could play important roles in the development and functions of these tissues.

Gap junction proteins exhibit early and specific expression during intramembranous bone formation in the developing chick mandible

The spatial and temporal expression of three closely related members of the connexin family of gap junction proteins (connexin42, Cx42; connexin43, Cx43; and connexin45, Cx45) was evaluated during bone formation in the mandibular process of the chick embryo. Mandibles of chick embryos from Hamburger and Hamilton stage 25 (approximately 5 days) through 19 days of development were dissected, serially sectioned and processed for immunocytochemical localization, employing site-specific anti-connexin antibodies. Our data revealed that (1) Cx43 was present throughout mandibular bone formation; (2) although it appeared to be associated with all bone cell types, Cx43 was concentrated in mesenchymal cells during the earliest stages in the osteogenic lineage; (3) most importantly, the localization of Cx43 at sites of bone formation appeared to precede the overt expression of the osteogenic phenotype; (4) by contrast, Cx45 was more restricted, spatially and temporally, in its distribution; (5) Cx42 expression was not detected in osteogenic tissue during mandibular bone formation. From all of the data obtained, Cx45 appeared to be associated with stages of bone formation characterized by the elaboration of matrix and the progressive expression of the differentiated osteogenic phenotype. Cx43 appeared to be associated with condensation of mesenchyme and the earliest stages of osteogenesis. Because of these associations, we propose that connexin expression may be necessary for the initiation of bone formation and the full expression of the osteogenic phenotype.

Ischemia-induced cellular redistribution of the astrocytic gap junctional protein connexin43 in rat brain

The distribution and levels of the astrocytic gap junction protein, connexin43 (Cx43) was analyzed in various regions of brain as a function of time after neuronal loss and consequent reactive gliosis induced by bilateral carotid occlusion in rats. In the striatum 2 days after induction of ischemia, immunostaining intensity for Cx43 increased in animals exhibiting mild to moderate striatal damage, whereas areas of reduced staining surrounded by elevated levels of Cx43 immunoreactivity were observed in animals with severe ischemic damage. Immunolabelling of glial cell bodies was evident in ischemic, but not normal, striatum. Similar, though less dramatic, changes were seen at 7 days post-ischemia. Compared with the fine punctate pattern of Cx43 staining seen in normal striatum, ischemic striatal areas contained large aggregates of punctate profiles. In the hippocampus, increased immunostaining was seen at 2 and 7 days post-ischemia and, unlike normal hippocampus, neurons in the CA3 pyramidal cell layer were surrounded by a network of Cx43-immunoreactive puncta at the latter survival time. Immuno-EM analysis of ischemic tissue revealed numerous immunolabelled gap junctions among astrocytic processes in the vicinity of degenerating neurons and elevated levels of intracellular Cx43 immunoreactivity in astrocytic processes and cell bodies. No differences in protein levels or phosphorylation states of Cx43 were detected in either hippocampus or striatum by Western blot analyses of ischemic and control tissue. These results suggest that astrocytes respond to an ischemic insult by reorganizing their gap junctions, that the qualitative nature of their response is dependent on the severity of neuronal damage or loss, and that a pool of Cx43 normally undetectable by immunohistochemistry may contribute to the ischemia-induced elevations of immunolabelling for this protein.

Colocalized alterations in connexin32 and cytochrome P450IIB1/2 by phenobarbital and related liver tumor promoters

Direct intercellular signal transduction is achieved by the passage of small molecules through gap junctions (GJ). Previous studies in our laboratory showed that the liver tumor promoter phenobarbital (PB) reversibly decreases the abundance of the GJ protein connexin32 (Cx32) in both preneoplastic-altered hepatic foci and centrolobular hepatocytes (M. J. Neveu et al., Cancer Commun., 2: 21-31, 1990). Because the inhibitory effects of PB on GJ intercellular communication are prevented by the nonspecific cytochrome P-450 inhibitor SKF-525A (J. E. Klauning, et al., Toxicol. Appl. Pharmacol., 102: 533-563, 1990), we investigated whether alterations in Cx32 are coincident with changes in the major PB-inducible cytochrome P-450, termed b/e or IIB1/2. Immunostaining of liver cryosections from rats fed dietary PB demonstrated that centrolobular hepatocytes that exhibit reduced Cx32 express enhanced cytochrome P450IIB1/2 protein. In contrast, no change in the periportal distribution of connexin26 immunoreactivity was found in PB-treated rats. In addition, rats were treated with the structurally related barbiturates pentobarbital, amobarbital, barbital, and barbituric acid. We found that the extent of the hepatic lobule occupied by coincident centrolobular alterations in Cx32 and P-450 staining correlates with the ability of the compounds to promote liver oncogenesis. To determine the molecular mechanisms responsible for the modification in Cx32 staining, we examined the mRNA and protein levels of Cx32 and P450IIB1/2 in total-tissue homogenates from PB-treated rats. Northern blotting demonstrated thatdietary PB dramatically induced P-450IIB1 mRNA, but the same RNA samples failed to show alterations in Cx32 steady-state transcripts. Consistent with these findings, the level of Cx32 protein in total liver homogenates did not change in rats chronically fed PB. Examination of Cx32 solubility in 20 mM NaOH demonstrated that PB treatment results in the generation of a NaOH-soluble form of Cx32 (i.e., 47 kDa). In addition, trypsinized paraffin-embedded liver sections from PB-treated rats exhibited diffuse cytoplasmic Cx32 staining that was restricted to centrolobular cells. Our results show that PB and related barbiturate tumor promoters reversibly down-regulate punctate Cx32 staining in centrolobular hepatocytes posttranslationally, possibly through modification(s) in the transport, assembly, and/or turnover of GJs.

Phosphorylated forms of connexin43 predominate in rat brain: demonstration by rapid inactivation of brain metabolism

The gap junction protein connexin43 (Cx43) has been reported to exist as several phosphorylated forms migrating at approximately 43 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis as well as an unphosphorylated 41-kDa form. In brain, Cx43 is expressed predominantly in astrocytes and is also expressed in several other cell types. Whereas the phosphorylated forms of Cx43 predominate in heart, several studies have indicated that high levels of the unphosphorylated form of Cx43 are present in brain. Various experiments in this report indicate that the 41-kDa molecular form in brain is a postmortem dephosphorylation product of phosphorylated Cx43. In rats killed by cranial high-energy microwave irradiation leading to rapid inactivation of brain metabolism, Cx43 in cerebral cortex was present almost exclusively as the 43-kDa phosphorylated form. Rapid dissection of brain followed by heat treatment or inclusion of phosphatase inhibitors during tissue homogenization also largely prevented the conversion of the 43- to the 41-kDa form. The 41-kDa species was generated after alkaline phosphatase digestion of the 43-kDa material obtained by immunoprecipitation from microwave-irradiated brain. Immunolabeling patterns and relative regional levels of Cx43 as seen by immunohistochemical and western blot detection were the same whether or not metabolism to the 41-kDa species was prevented. In developing rat brain, Cx43 levels in frontal cortex and brainstem increased with age, but the degree of dephosphorylation of the 43- to the 41-kDa form was greater at earlier ages in the brainstem. It appears that brain contains a phosphatase that may be involved in modulating the phosphorylation state of Cx43 and thus may regulate intercellular communication via astrocytic gap junctions.

Kainic acid induced alterations in antibody recognition of connexin43 and loss of astrocytic gap junctions in rat brain

Intracerebral administration of kainic acid (KA) in rats was previously shown to abolish immunohistochemical labelling for the astrocytic gap junction protein connexin43 (Cx43) at sites depleted of neurons (Vukelic et al: Neurosci Lett 130:120-124, 1991). This response of Cx43 has now been further investigated with a number of different sequence-specific anti-Cx43 antibodies. At lesion sites in the thalamus, striatum, and hippocampus examined immunohistochemically with an antibody against amino acids (aa's) 346-363 in the Cx43 sequence, the antibody used in the earlier study, Cx43-immunoreactivity was increased 5 h after KA injections, absent by 24 h and for up to 2 weeks post-injection, and began to return to less than normal levels by 2 to 3 weeks post-injection. Analyses of KA lesion sites with antibodies against other sequences of Cx43 (amino acids 283-298, 253-270, 241-260, 113-123, and 49-61) revealed not only the presence but in some cases an increased density of Cx43 immunoreactivity after a survival time of 1 week. Immunolabelling patterns at these sites consisted of relatively large, coarse profiles rather the fine punctate labelling typically seen in sections of normal brain. In homogenates of KA-injected striatum analyzed by Western blots, Cx43 was detected at near normal or slightly increased levels at various survival times examined. The 43 kDa phosphorylated form of Cx43 and its faster migrating 41 kDa dephosphorylated form which is generated post-mortem by a brain phosphatase were both present after standard methods of tissue preparation for Western blot analysis, while only the 43 kDa form was present in normal and KA-injected striatum after inactivation of brain metabolism by focused cranial microwave irradiation. Ultrastructural investigations of lesions sites within the thalamus revealed a virtual absence of astrocytic gap junctions. These results demonstrate that Cx43 levels initially increase after intracerebral KA treatment, that its molecular organization in resident astrocytes is altered such that epitopes that are normally accessible to antibody are hidden while those that may be hidden or relatively inaccessible are exposed, and that this molecular alteration in Cx43 is associated with loss of astrocytic gap junctions.

Modulation of gap junction-mediated intercellular communication in embryonic chick mesenchyme during tissue remodeling in vitro

Gap junction-mediated intercellular communication was analyzed in a model system in which tissue necrosis and remodeling could be modulated. This in vitro system, previously used for analysis of epithelial-mesenchymal tissue interaction, was modified to permit analysis of the presence and extent of intercellular communication by monitoring intercellular transfer of the microinjected fluorescent dye, Lucifer Yellow. Light and transmission electronmicroscopy were employed to correlate the presence and degree of gap junctional communication (coupling) with tissue morphology. Digital image analysis was used to determine cell density and mitotic indices within the outgrowths of explants. Our results indicated that cell communication in outgrowths adjacent to necrotic foci within an explant was minimal or absent. Cell-coupling in outgrowths adjacent to a compartment of viable mesenchyme was significantly higher - equivalent to unseparated control cultures. A time-course study demonstrated correlation of increased levels of cell-coupling in outgrowths with the level of tissue remodeling within an explant. Our conclusions from these studies are that embryonic mesenchymal cell populations may be selectively uncoupled as a result of alterations in the microenvironment produced by a proximate impaired cell population. It is proposed that endogenous factors in the microenvironment ("wound signals"), emanating from impaired cell populations, regulate gap junction-mediated intercellular communication in adjacent viable tissue. Normal, unimpaired populations of cells surrounding an area of injury are thereby isolated from the effects of a potentially toxic environment. This could serve as a protective function in development and may represent, in a more general sense, part of the repertoire of events associated with tissue repair and remodeling.

Multiple mechanisms are responsible for altered expression of gap junction genes during oncogenesis in rat liver

Although several abnormalities in gap junction (GJ) structure and/or function have been described in neoplasms, the molecular mechanisms responsible for many of the alterations remain unknown. The identification of a family of GJ proteins, termed connexins, prompted this study of connexin32 (Cx32), connexin26 (Cx26) and connexin43 (Cx43) expression during rat hepatocarcinogenesis. Using antibody, cDNA and cRNA probes, we investigated connexin mRNA and protein expression in preneoplastic and neoplastic rat livers. In normal liver, Cx32 is expressed in hepatocytes throughout the hepatic acinus, Cx26 is restricted to periportal hepatocytes, and Cx43 is expressed by mesothelial cells forming Glisson's capsule. Most preneoplastic altered hepatic foci generated by diethylnitrosamine (DEN) initiation and either phenobarbital (PB) or 2,3,7,8-dichlorodibenzo-p-dioxin (TCDD) promotion exhibited decreased Cx32 or increased Cx26 staining. Foci from either protocol failed to display Cx43 immunoreactivity. In the majority of PB-promoted foci, Cx32 immunoreactivity decreased independently of changes in mRNA abundance. Continuous thymidine labeling, following cessation of PB promotion, showed that downregulation of Cx32 staining is reversible in foci that are promoter-dependent for growth, but irreversible in lesions that are promoter-independent for growth. Hepatic neoplasms from rats initiated with DEN and promoted with PB or TCDD also displayed modified connexin expression. While all 24 neoplasms studied were deficient in normal punctate Cx32 and Cx26 staining, altered cellular localization of these proteins was apparent in some tumors. Immunoblotting of crude tissue extracts revealed that neoplasms with disordered Cx32 staining showed immunoreactive bands with altered electrophoretic mobility. These observations show that hepatomas may downregulate Cx32 expression through changes in the primary structure of Cx32 or by post-translational modifications. Northern blotting of total tumor mRNAs failed to demonstrate consistent changes in the abundance of Cx32, Cx26 or Cx43 transcripts. Some tumors expressed steady-state transcripts without observable immunoreactivity, indicating that some hepatomas downregulate connexin immunoreactivity independently of mRNA abundance. Increased levels of Cx43 mRNA and protein were found in several neoplasms, but immunostaining was always localized to nonparenchymal cells. Areas of bile duct proliferation and cholangiomas displayed Cx43 staining, whereas, cholangiocarcinomas were deficient in immunoreactivity. These findings show that alterations in the expression of connexins, by either downregulation or differential induction, represent common modifications during hepatocarcinogenesis. Although our results imply that connexins represent useful markers for the boundary between tumor promotion and progression, preneoplastic and neoplastic rat hepatocytes fail to use a common mechanism to modify connexin expression.

On the mechanisms of cell uncoupling induced by a tumor promoter phorbol ester in clone 9 cells, a rat liver epithelial cell line

It is known that in Clone 9 (C9) cells, intercellular gap junctional communication (IGJC) is rapidly blocked by the tumor promoter phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), but it recovers spontaneously a few hours later and becomes refractory to TPA (Yada et al., J. Membr. Biol. 88, 217-232 (1985)). We now report that gap junctions between C9 cells contain at least two junctional proteins, connexin26 (Cx26) and connexin43 (Cx43), and that the TPA-induced changes in IGJC correlate temporally to changes in the state of phosphorylation of Cx43. The latter changes were prevented by inhibition of protein kinase C. Phosphoamino acid analysis and two-dimensional tryptic peptide maps of 32P-labeled Cx43 showed that during the TPA-induced phosphorylation at least two of the phosphorylated forms of Cx43 were differentially phosphorylated in seryl residues as compared to control. TPA induced a drastic reduction in junctional conductance as well as a redistribution of unitary gap junction channel event sizes seen in control cells. These changes were associated with retrieval of Cxs from the plasma membrane. Reappearance of gap junctions formed by Cx43 but not by Cx26 accounted for the spontaneous recovery in IGJC. It is proposed that gap junctions between C9 cells contain two types of channels each formed by Cx43 or Cx26 and that they are differentially affected during the action of TPA.

Liver fat-storing cell clones obtained from a CCl4-cirrhotic rat are heterogeneous with regard to proliferation, expression of extracellular matrix components, interleukin-6, and connexin 43

BACKGROUND

Immunocytochemical analysis of liver has revealed that fat-storing cells (FSC) are heterogeneous with regard to vitamin A content, staining for cytokeratins, desmin, and vimentin and the cytoskeletal protein alpha-smooth muscle actin. Since fat-storing cells play an important role in collagen deposition in normal and cirrhotic liver, we considered it important to study whether fat-storing cells were heterogeneous with regard to cell proliferation, expression of mRNAs coding for cytokines interleukin-6 (IL-6) and transforming growth factor-beta (TGF-beta), and extracellular matrix components alpha 1(I), alpha 1(III), alpha 1(IV) procollagens, laminin B1 chain and fibronectin.

EXPERIMENTAL DESIGN

We used a FSC line (CFSC) that was developed in our laboratory after spontaneous immortalization of a primary culture of fat-storing cells that were obtained from the liver of a CCl4-cirrhotic rat (Lab. Invest. 65:644-653, 1991). The cells were cloned by limiting dilution and have been maintained in culture for over 3 years without appreciable changes in the parameters investigated.

RESULTS

In this communication we report the characterization of 4 of the clones obtained. We show that they are heterogeneous with regard to proliferation index, expression of alpha 1(I), alpha 1(III) and alpha 1(IV) procollagen, IL-6 and TGF-beta mRNAs. The clones also differ in their response to IL-6. We also showed that clones are coupled through functional gap junctions but that they are heterogeneous with regard to the expression of the gap junction protein connexin 43.

CONCLUSIONS

We suggest that clonal heterogeneity of FSC may occur in vivo. Since each of the clones expresses a unique phenotype, these FSC clones could be excellent models to study the role of defined extracellular matrices on the expression of liver specific genes by cultured hepatocytes.

Connexin43 in rat pituitary: localization at pituicyte and stellate cell gap junctions and within gonadotrophs

Immunohistochemical methods were employed to investigate the cellular and ultrastructural localization of the gap junction protein connexin43 (Cx43) in rat pituitary. Western blots of pituitary homogenates probed with anti-Cx43 antibodies showed the presence of Cx43 in both anterior and posterior pituitary lobes. By light microscopy (LM), Cx43-immunoreactive (Cx43-IR) puncta were found in all areas of the posterior lobe, but at greater concentrations in peripheral regions of this structure. By electron microscopy (EM), immunogold labelling for Cx43 was seen at gap junctions between thin cytoplasmic processes of pituicytes. No immunoreactivity was detected in the intermediate lobe. The anterior lobe contained puncta similar to but more sparsely scattered than those in the posterior lobe, and by EM analysis these were demonstrated to correspond to labelled gap junctions between stellate cells. In addition, anti-Cx43 antibodies produced intracellular labelling in a small percentage of endocrine cells, which were distributed throughout the anterior lobe and determined by double immunostaining methods to be cells containing luteinizing hormone. By EM, labelling within these cells was associated with predominantly large secretory granules and other loosely organized organelles. The results indicate that gap junctions in the pituitary are composed of Cx43 and that this or a related protein may have a novel intracellular function within gonadotrophs.

Connexin expression in the developing avian cardiovascular system

Recent observations have suggested that the patterns of expression of the gap junction protein connexin43 in the developing cardiovascular system of the avian embryo diverge significantly from the patterns previously seen in mammalian species. Therefore, a detailed analysis of connexin43 expression in the chicken embryo was performed by use of immunofluorescent localization with two different connexin43-specific antipeptide antibodies as well as Western and Northern blot analysis. Connexin43 protein was not detected in the avian myocardium, the venous system, or the smaller vessels of the arterial system. Rather, it was limited exclusively to the vessels of the arterial outflow tract in a concentric pattern that became evident by embryonic day 8. Double staining with anti-alpha-smooth muscle actin and connexin43 demonstrated colocalization in the media of outflow tract vessel walls. The developmental expression of connexin43 was found to mirror the spatial patterning of secondary actin; connexin43, however, preceded the expression of secondary actin by a period of 1-2 days. In contrast, antibodies to a related gap junction protein (connexin42) revealed an absence of immunostaining in the avian outflow tract. Double staining with anti-connexin42 and anti-A-cell adhesion molecule (specific for avian intercalated discs) demonstrated colocalization between cardiac myocytes, indicating that connexin42 is a constituent of avian myocardial gap junctions. In light of these findings, developmental expression of differing myocardial connexins may reconcile previous studies showing different physiological properties of avian and mammalian cardiac gap junctions.

Gap junctions formed of connexin43 are found between smooth muscle cells of human corpus cavernosum

Despite sparse autonomic innervation, the smooth muscle cells of the corpus cavernosum relax and contract synchronously to achieve penile erection and flaccidity. As with other smooth muscle cell types, the excitation process in the corpora is presumably propagated through gap junctions to allow the diffusion of current-carrying ions and second messenger molecules from cell to cell. Using both molecular and immunocytochemical techniques, we have identified gap junctions between human corporal smooth muscle cells in situ and in culture. Northern analyses demonstrated that corporal smooth muscle cells express the gap junction protein connexin43, but not connexin26 mRNA. Immunoblots showed the presence of connexin43 isoforms, whereas connexin32 was not detected. Immunocytochemical studies in cultured cells identified prominent connexin43 immunoreactive puncta between cells, as well as within the cytoplasm. In addition, gap junction membranes both in situ and in culture were labelled in thin section by anti-connexin43 antibodies using the immunogold technique. We conclude that the presence and distribution of gap junctions in this sparsely innervated tissue may provide an important mechanism of intercellular communication among the smooth muscle cells, and thus play a major role in coordinating tissue contraction and relaxation.

Expression of connexins in the developing olfactory system of the mouse

To gain insight into the function of gap junctions' connexin43, connexin32 and connexin26 in a neural structure that retains neuronal turnover capacities throughout adulthood, the expression of these molecules has been investigated in the developing and adult olfactory system by immunocytochemical and biochemical methods. Connexin43 was detectable from the olfactory placode stage. During early embryonic development, the levels of connexin43 expression remained low. An increase in the expression of this connexin occurred perinatally. Expression of connexin43 became very high during the postnatal stages and adulthood. Electron microscopy (EM) immunocytochemistry of the olfactory system showed connexin43 expression in non-neuronal cells. Strong regional differences in the expression of connexin43 in the olfactory epithelium were observed. No apparent relationship between connexin43 expression and turnover activity of olfactory neurons was detected. Western blots of olfactory tissues revealed the presence of three different isoforms of connexin43. Connexin32 was detected in the olfactory bulb at late postnatal stages including adulthood. Connexin32 was observed on some cells tentatively identified as oligodendrocytes. Connexin26 was localized onto leptomeninges. Some immunofluorescence was also obtained in the periglomerular region and in the subependymal layer of the bulb. Northern blot analysis revealed the presence of mRNA of connexin32 and connexin26 in the adult olfactory system. Our results substantiate the cell specific expression of these three types of connexins and they document the primary of connexin43 in olfactory tissues. Moreover, our findings indicate that although expression of connexin43 in the olfactory system is developmentally regulated, it is not directly associated with the neuronal cell turnover of the olfactory epithelium.

Low molecular weight GTP-binding proteins in cardiac muscle. Association with a 32-kDa component related to connexins

Low molecular weight GTP-binding proteins and their cellular interactions were examined in cardiac muscle. Heart homogenate was separated into various subcellular fractions by differential and sucrose density gradient centrifugation. Various fractions were separated by sodium dodecyl sulfate-gel electrophoresis, blotted to nitrocellulose, and GTP-binding proteins detected by incubating with [alpha-32]GTP. Three polypeptides of M(r) 23,000, 26,000, and 29,000 were specifically labeled with [alpha-32P]GTP in all the fractions examined and enriched in sarcolemmal membranes. The 23-kDa polypeptide was labeled to a higher extent with [alpha-32P]GTP than the 26- and 29-kDa polypeptides. A polypeptide of M(r) 40,000 was weakly labeled with [alpha-32P]GTP in the sarcolemmal membrane and tentatively identified as Gi alpha by immunostaining with anti-Gi alpha antibodies. Cytosolic GTP-binding proteins were labeled with [alpha-32P]GTP and their potential sites of interaction investigated using the blot overlay approach. A polypeptide of 32 kDa present in sarcolemmal membranes, intercalated discs, and enriched in heart gap junctions was identified as a major site of interaction. The low molecular weight GTP-binding proteins associated with the 32-kDa polypeptide through a complex involving cytosolic components of M(r) 56,000, 36,000, 26,000, 23,000, and 12,000. A monoclonal antibody against connexin 32 from liver strongly recognized the 32-kDa polypeptide in heart gap junctions, whereas polyclonal antibodies only weakly reacted with this polypeptide. The low molecular weight GTP-binding proteins associated with a 32-kDa polypeptide in liver membranes that was also immunologically related to connexin 32. These results indicate the presence of a subset of low molecular weight GTP-binding proteins in a membrane-associated and a cytoplasmic pool in cardiac muscle. Their association with a 32-kDa component that is related to the connexins suggests that these polypeptides may be uniquely situated to modulate communication at the cell membrane.

Differential anatomical and cellular patterns of connexin43 expression during postnatal development of rat brain

We have shown previously that connexin43 in the adult rat central nervous system (CNS) is predominantly localized at astrocytic gap junctions. Here we document immunohistochemically the emergence of connexin43-immunoreactive (connexin43-IR) structures and the regional patterns of connexin43 expression during postnatal maturation of the rat brain. On Western blots, connexin43 was detected in brain samples at postnatal day (P) 5, the earliest age studied. Immunohistochemically, most brain regions displayed a characteristic sequence of transient immunoreactive profiles that ultimately gave rise to the uneven distribution of the protein seen in adults. Generally, brains at P1-P5 exhibited long, fibrous connexin43-IR elements which were identified as radial glial cells. This fibrous immunostaining was considerably diminished at P5 and was replaced by short immunoreactive processes which predominated up to P10. These processes had a stellate appearance, emanated from partially stained astrocytic cell bodies and were heterogeneously distributed throughout the developing brain. By P15, there occurred only punctate immunolabelling similar to that seen in adult brain. Some brain regions including the amygdaloid complex, septohypothalamic nucleus, preoptic hypothalamus, zona incerta, ependyma and subfornical organ were exceptional in that they displayed adult immunostaining patterns at early postnatal ages suggesting a precocious maturation of gap junctions in these areas. We conclude that the highly heterogeneous distribution of connexin43-immunoreactivity among defined nuclear structures in adult brain does not reflect an antecedent requirement for connexin43 in early brain morphogenesis, but rather is related to the development of neuronal activity, the establishment of functional circuitry and the contribution of astrocytic gap junctions to glial metabolic coupling and potassium spatial buffering in the mature CNS.

Gap junction distribution is altered between cardiac myocytes infected with Trypanosoma cruzi

Conduction disturbances frequently accompany both acute and chronic Chagas' disease. To explore the possibility that changes in gap junction distribution or abundance might play a role in these disturbances, we have investigated intercellular communication between rat neonatal cardiac myocytes in cultures infected with Trypanosoma cruzi. Contractile activity of infected cells was characterized by regional asynchrony within the culture as well as by irregular contraction patterns. Junctional conductance between infected cell pairs was found to be significantly lower than in uninfected cell pairs, and the rapidity and extent of intercellular transfer of the dye lucifer yellow was markedly reduced between infected cells. Immunocytochemical studies demonstrated that the parasitic infection significantly decreased connexin43 expression at junctional membrane regions, correlating with the detected functional uncoupling. These findings of reduced gap junction abundance and function in trypanosome-infected cells may provide important insight into the pathogenesis of the cardiac arrhythmias that attend Chagas' disease.

Connexin43 in MDCK cells: regulation by a tumor-promoting phorbol ester and Ca2+

Prior to confluence, cultures of Madin Darby canine kidney (MDCK) cells expressed gap junctional communication, as assessed by fluorescent dye transfer, as well as relatively high levels of an anti-connexin43 immunoreactive component referred to as connexin43 (Cx43). After confluence, dye coupling and levels of Cx43 were dramatically reduced. Immunofluorescence analysis of the distribution of Cx43 in subconfluent cultures showed punctate labeling on the plasma membrane at regions of cell apposition and a more diffuse labeling in perinuclear regions. Western blots of total cell homogenates showed that the dephosphorylated form of Cx43 was more abundant than the phosphorylated forms. Phosphorylation of Cx43 was not significantly affected by 8-Bromo-cAMP or 8-Bromo-cGMP. However, 12-O-tetradecanoylphorbol-13-acetate (TPA) inhibited dye coupling and induced an increase in the amount of phosphorylated forms of Cx43 at the expense of the dephosphorylated form. This effect occurred as rapidly as 5 min after TPA treatment without apparent changes in distribution of Cx43 or cell morphology. These results suggest that second messenger pathways involving protein kinase C, but not cAMP- or cGMP-dependent protein kinase, led to changes in electrophoretic mobility of Cx43, revealed by Western blot, consistent with an alteration in the state of phosphorylation of the gap junction protein. Treatments with staurosporine, a protein kinase inhibitor, or okadaic acid, a protein phosphatase inhibitor, either alone or in combination with TPA, indicated that the abundance of the dephosphorylated form of Cx43 in MDCK cells was due to low kinase activity. It was also found that lowering the concentration of extracellular Ca2+, which reduced cell contact, did not affect the abundance, the state of phosphorylation, or the TPA-induced phosphorylation of Cx43. These results suggest that neither extracellular Ca2+ nor cell contact is required for basal or TPA-induced phosphorylation of Cx43.

Heterogeneity in gap junction expression in astrocytes cultured from different brain regions

Heterogeneity among astrocytes suggests that their role in the central nervous system is more complex than is commonly recognized. This paper describes just such a functional difference, comparing gap junctions in astrocytes derived from two brain regions. Astrocytes, both in situ and in culture, employ gap junctions as a means of intercellular communication. Recent evidence utilizing cultured rat cortical and striatal astrocytes has shown that these channels consist of subunits of connexin 43, the same protein as that composing cardiac gap junctions. Here we report that astrocytes cultured from neonatal rat hypothalamus contain a greater number of functional channels than astrocytes from the striatum, a difference reflected in both connexin 43 protein and mRNA. Specifically, in hypothalamic astrocytes the level of connexin 43 protein was approximately four times that found in comparable cultures from the striatum, as determined by immunoblotting. Complementary results from immunocytochemical experiments using an antibody specific for connexin 43 reveal significantly greater fluorescence in astrocytes cultured from the hypothalamus as compared to those from the striatum. Northern blot analysis showed that connexin 43 mRNA levels were also approximately 4-fold greater in the hypothalamic cultures, consistent with the difference seen by immunoblotting. Finally, dye coupling studies using confluent cultures consistently showed that within 1 min Lucifer Yellow injected into striatal astrocytes spread to immediately surrounding cells while in hypothalamic astrocytes dye often spread to apparent third or fourth order neighbors within the same time period. Thus, the higher level of connexin 43 expression seen in hypothalamic astrocytes results in cells with greater numbers of functional channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative immunohistochemical and biochemical correlates of connexin43 localization in rat brain

We have shown by immunohistochemical methods that the gap junction protein connexin43 is heterogeneously distributed in rat brain (Yamamoto et al: J Comp Neurol 302:853, 1990). Here we have compared quantitatively the relative amount of connexin43 detected on Western blots of seven central nervous system (CNS) regions with the density of connexin43-immunoperoxidase reactivity in these regions. As has been observed on Western blots of several cell types, homogenates of these CNS regions contained two forms of connexin43, its dephospho form with an apparent mobility of approximately 41 kDa and its approximately 43 kDa phosphorylated form. While the relative quantities of connexin43 varied considerably among the brain regions, the ratio of the 43/41 kDa forms, 0.71, was relatively uniform (correlation coefficient, r = 0.92). Sections of brain processed for connexin43-immunolocalization by the peroxidase-antiperoxidase (PAP) method showed that chromogen deposition was linear with incubation time in reaction medium. Optical density of tissue connexin43-immunoreactivity in each of the seven areas plotted against the density of connexin43 bands on Western blots gave a correlation coefficient of r = 0.90. Connexin43-immunoreactivity had a similar appearance in sections processed by PAP or immunofluorescence procedures and consisted of isolated or aggregates of puncta.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative immunoassay of total cellular GAP junction protein connexin32 during liver regeneration using antibodies specific to the COOH-terminus

A radioimmunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA) were used to determine relative concentrations of liver connexin32 (CX32) in rats. The RIA and ELISA utilize synthetic peptides corresponding to regions of the carboxyl-terminus and antibodies raised in rabbits against these peptides. Assuming that affinities of antisera are similar for peptide and native CX32, total cellular CX32 was found to exceed the amount of gap junction protein at the cell surface calculated from morphometric analyses by 1.5-2.0 fold. This finding raises the possibility that some of the protein is present in cytoplasmic compartments or as occult precursors in the plasma membrane. Studies of CX32 content in regenerating rat liver support this conclusion and show a time course of loss and recovery of CX32 that agrees with those reported in studies using other techniques.

Biochemical and ultrastructural evidence for the association of basic fibroblast growth factor with cardiac gap junctions

Basic fibroblast growth factor (bFGF) is a ubiquitous and multifunctional polypeptide that is believed to have a role in tissue repair and to act as a morphogen in embryonic development. Here, we have used immunohistochemical and biochemical methods with antibodies directed against the amino-terminal domain of bFGF, designated IS2, which recognize native and denatured bFGF, to demonstrate that in addition to its known intracellular and extracellular localization in heart, bFGF is also associated with cardiomyocyte gap junctions. In tissue sections, IS2 labeled regions of intercalated discs, producing an immunofluorescence pattern virtually indistinguishable from that obtained with antibodies against the heart gap junction protein connexin-43. By electron microscopy, gap junctions but not other regions of plasma membrane were heavily immunolabeled with this antibody. By solid phase immunoassay, bFGF was found to be more concentrated in a fraction enriched in cardiac gap junctions than in whole sarcolemmal preparations. Finally, an 18-kDa protein was recognized by several different antibodies specific for bFGF on Western blots of heart subcellular fractions enriched in gap junctions. We suggest that bFGF-like peptides are either an integral part of, or exist in close association with, cardiac gap junctions and thus may play a role in modulating gap junctional intercellular communication.

Depletion of connexin43-immunoreactivity in astrocytes after kainic acid-induced lesions in rat brain

Recent studies have established that the gap junction protein connexin43 is a major structural component of gap junctions between astrocytes in rat brain. Here, we investigated by immunohistochemical methods the effect of kainic acid-induced neuronal degeneration on connexin43 expression by astrocytes. Stereotaxic injections of kainic acid into the thalamus were found to cause a near total depletion of connexin43-immunoreactivity at the lesion site. Areas depleted of connexin43 corresponded to those exhibiting substantial neuronal loss and intense gliosis. These results implicate a neuronal contribution to the regulation of connexin43 expression by astrocytes and, hence, to local control of the potassium spatial buffering capacity afforded by astrocyte gap junctions.