Complex formation between junctin, triadin, calsequestrin, and the ryanodine receptor. Proteins of the cardiac junctional sarcoplasmic reticulum membrane

Several key proteins have been localized to junctional sarcoplasmic reticulum which are important for Ca2+ release. These include the ryanodine receptor, triadin, and calsequestrin, which may associate into a stable complex at the junctional membrane. We recently purified and cloned a fourth component of this complex, junctin, which exhibits homology with triadin and is the major 125I-calsequestrin-binding protein detected in cardiac sarcoplasmic reticulum vesicles (Jones, L. R., Zhang, L., Sanborn, K., Jorgensen, A. O., and Kelley, J. (1995) J. Biol. Chem. 270, 30787-30796). In the present study, we have examined the binding interactions between the cardiac forms of these four proteins with emphasis placed on the role of junctin. By a combination of approaches including calsequestrin-affinity chromatography, filter overlay, immunoprecipitation assays, and fusion protein binding analyses, we find that junctin binds directly to calsequestrin, triadin, and the ryanodine receptor. This binding interaction is localized to the lumenal domain of junctin, which is highly enriched in charged amino acids organized into "KEKE" motifs. KEKE repeats are also found in the common lumenal domain of triadin, which likewise is capable of binding to calsequestrin and the ryanodine receptor (Guo, W., and Campbell, K. P. (1995) J. Biol. Chem. 270, 9027-9030). It appears that junctin and triadin interact directly in the junctional sarcoplasmic reticulum membrane and stabilize a complex that anchors calsequestrin to the ryanodine receptor. Taken together, these results suggest that junctin, calsequestrin, triadin, and the ryanodine receptor form a quaternary complex that may be required for normal operation of Ca2+ release.

Cellular redistribution of protein tyrosine phosphatases LAR and PTPsigma by inducible proteolytic processing

Most receptor-like protein tyrosine phosphatases (PTPases) display a high degree of homology with cell adhesion molecules in their extracellular domains. We studied the functional significance of processing for the receptor-like PTPases LAR and PTPsigma. PTPsigma biosynthesis and intracellular processing resembled that of the related PTPase LAR and was expressed on the cell surface as a two-subunit complex. Both LAR and PTPsigma underwent further proteolytical processing upon treatment of cells with either calcium ionophore A23187 or phorbol ester TPA. Induction of LAR processing by TPA in 293 cells did require overexpression of PKCalpha. Induced proteolysis resulted in shedding of the extracellular domains of both PTPases. This was in agreement with the identification of a specific PTPsigma cleavage site between amino acids Pro821 and Ile822. Confocal microscopy studies identified adherens junctions and desmosomes as the preferential subcellular localization for both PTPases matching that of plakoglobin. Consistent with this observation, we found direct association of plakoglobin and beta-catenin with the intracellular domain of LAR in vitro. Taken together, these data suggested an involvement of LAR and PTPsigma in the regulation of cell contacts in concert with cell adhesion molecules of the cadherin/catenin family. After processing and shedding of the extracellular domain, the catalytically active intracellular portions of both PTPases were internalized and redistributed away from the sites of cell-cell contact, suggesting a mechanism that regulates the activity and target specificity of these PTPases. Calcium withdrawal, which led to cell contact disruption, also resulted in internalization but was not associated with prior proteolytic cleavage and shedding of the extracellular domain. We conclude that the subcellular localization of LAR and PTPsigma is regulated by at least two independent mechanisms, one of which requires the presence of their extracellular domains and one of which involves the presence of intact cell-cell contacts.

Endoplasmic reticulum to Golgi trafficking in multinucleated skeletal muscle fibers

The organization of membrane trafficking between endoplasmic reticulum and Golgi within multinucleated muscle fibers was analyzed. We found that markers for the compartment involved in endoplasmic reticulum to Golgi trafficking exhibited perinuclear as well as interfibrillar localization. Furthermore, these markers showed prominent colocalization with microtubules. To analyze membrane trafficking, we followed the temperature-controlled transport of the G protein of the mutant vesicular stomatitis virus, tsO45, in isolated myofibers. Perinuclear and cross-striated staining were seen at 39 degrees C, while at 15 degrees C a diffuse staining component appeared along a subset of interfibrillar microtubules. At 20 degrees C, bright Golgi spots were seen to be associated with microtubules that appeared as circumnuclear rings and longitudinal bundles. Beneath the motor end plate, however, the organization of the Golgi elements and microtubules was found to be distinctive. Retrograde trafficking induced by brefeldin A resulted in the disappearance of the Golgi spots throughout the myofibers and the appearance of staining along microtubules. Thus, interfibrillar membranes seem to be active in protein export, and trafficking between endoplasmic reticulum and Golgi elements occurred throughout the myofibers. The results suggest that microtubules served as tracks for the two-way trafficking between the endoplasmic reticulum and the Golgi compartment.

Differential distribution of the tight-junction-associated protein ZO-1 isoforms alpha+ and alpha- in guinea pig Sertoli cells: a possible association with F-actin and G-actin

To elucidate the significance of alpha- and alpha+ isoforms of the tight-junction-associated protein ZO-1 in Sertoli cell tight junction regulation, taking into consideration that different isoforms are expressed in cells with different junctional morphologies, we investigated whether alpha- and alpha+ are differentially associated with junctions forming the continuous occluding zonules responsible for the blood-testis barrier, and/or with junctions forming the focal discontinuous occluding zonules. In addition, since Sertoli cells contact Sertoli cells and germ cells, we investigated whether each isoform is differentially associated with distinct classes of germ cells. Our immunoblot analyses of isolated seminiferous tubules, using affinity-purified polyclonal antibodies recognizing rat and human alpha- and alpha+, showed that guinea pig testis contained the two ZO-1 isoforms initially described in rat and human kidneys, and that alpha+ and alpha- were predominantly expressed during puberty and adulthood, respectively, indicating that alpha+ was predominant during periods of increased junction assembly/disassembly. We used the same antibodies and immunoperoxidase labeling on fetal, neonatal, pubertal, and adult guinea pig testes sections. Both isoforms were expressed at the site of Sertoli cell-Sertoli cell and Sertoli cell-germ cell junctions in the seminiferous epithelium, before and after birth, and both were localized in continuous and in discontinuous tight junctions. However, the distribution of alpha- and alpha+ was not the same in different locations of the tight junctions. Only alpha- was incorporated into junctions joining the Sertoli cells to all classes of germ cells. The alpha+ involved junctions joining Sertoli cells to particular classes of germ cells, suggesting that Sertoli cell expression of ZO-1 isoforms could be regulated by unique germ cell-Sertoli cell contacts. Conversely, we found a correspondence between the distribution of F-actin and ZO-1alpha+, indicating that the spatial organization of the subsurface actin accompanying cell junctions may affect alpha+/alpha(-)-plasma membrane association.

Evidence that beta1 integrins in keratinocyte cell-cell junctions are not in the ligand-occupied conformation

Integrins are a family of heterodimeric cell surface molecules that function as adhesion receptors in cell-cell and cell-extracellular matrix contact. Integrins of the beta1 family are found on keratinocytes clustered at sites of cell-cell junctions both in culture and in normal skin. The possibility that these integrins function in cell-cell adhesion has been both supported and refuted in recent conflicting publications. Rather than testing further for the presence or absence of an interaction, we present evidence to show that beta1 integrins in keratinocyte cell-cell junctions are in the non-ligand-occupied conformation. We transfected keratinocytes with a construct that expresses a chimeric cell surface molecule containing the integrin beta1 cytoplasmic tail. This chimera is thought to mimic the ligand-occupied receptor and has previously been shown to be actively localized to focal adhesions in fibroblasts. We find that keratinocytes are also able to localize this chimera in focal adhesions but do not localize it to areas of cell-cell junctions. A monoclonal anti-beta1 antibody that has been previously shown to preferentially recognize ligand-occupied beta1 receptors was used to stain keratinocytes. This antibody showed staining of focal adhesions, with little or no staining of cell-cell junctions. In contrast, four other anti-beta1 antibodies showed strong, preferential staining at cell-celljunctions. Double staining confirmed that both the conformation-specific monoclonal antibody and a pan-beta1 antibody were capable of recognizing the same focal adhesions. Taken together, these data indicate that integrins in cell- cell junctions of keratinocytes are in the non-ligand-occupied conformation. Although we do not directly prove the absence of an integrin-integrin interaction at this site, we show that any such interaction does not induce the ligand-occupied conformation and, therefore, is less likely to play a major role in cytoskeletal re-organization or signal transduction.

Negative regulation of Armadillo, a Wingless effector in Drosophila

Drosophila Armadillo and its vertebrate homolog beta-catenin play essential roles both in the transduction of Wingless/Wnt cell-cell signals and in the function of cell-cell adherens junctions. Wingless and Wnts direct numerous cell fate choices during development. We generated a mutant protein, Armadillo(S10), with a 54 amino acid deletion in its N-terminal domain. This mutant is constitutively active in Wingless signaling; its activity is independent of both Wingless signal and endogenous wild-type Armadillo. Armadillo's role in signal transduction is normally negatively regulated by Zeste-white 3 kinase, which modulates Armadillo protein stability. Armadillo(S10) is more stable than wild-type Armadillo, suggesting that it is less rapidly targeted for degradation. We show that Armadillo(S10) has escaped from negative regulation by Zeste white-3 kinase, and thus accumulates outside junctions even in the absence of Wingless signal. Finally, we present data implicating kinases in addition to Zeste white-3 in Armadillo phosphorylation. We discuss two models for the negative regulation of Armadillo in normal development and discuss how escape from this regulation contributes to tumorigenesis.

Modulation of alpha 6/beta 1 integrin expression during differentiation of F9 murine embryonal carcinoma cells to parietal endoderm

In vitro differentiation of the murine embryonal carcinoma (EC) cell line F9 parallels that of the early blastocyst, where visceral (VE) and parietal endoderm (PE) diverge from a common precursor, the primitive endoderm. This differentiation pathway is induced by retinoic acid (RA) and dibutyryl cyclicAMP (dcAMP) and is accompanied by progressive and dramatic changes in cell morphology and functions. Within 7 days of treatment the cells organize their cytoskeleton and synthesize large amounts of extracellular matrix proteins, becoming fully differentiated migratory cells; all these changes are likely to involve integrins expression and organization. We have investigated the changes in beta 1 integrin expression, its maturation, and organization on the cell surface in association with alpha 6, during the transition from undifferentiated F9 stem cells to migrating PE cells. By Western blotting and immunoprecipitation we showed a gradual decrease in the amount of the beta 1 subunit on the cell surface and a parallel progressive accumulation of immature protein, indicating that the control of beta 1 expression during F9 cells differentiation occurs first at post-translational level and then at the level of transcription. Moreover, the induction of differentiation produces a marked decrease of alpha 6B and its association to a high molecular weight protein, while alpha 6A level increases. By immunofluorescence we found that upon differentiation there is a relocation of the beta 1 and alpha 6B integrin subunits from cell-cell contacts to focal contacts where they colocalize with vinculin. On the contrary alpha 6A, weakly present in F9 stem cells, is present in the focal contacts of PE cells and along the stress fibers. We suggest different roles for the two alpha 6 isoforms.

Retention of functional characteristics by bovine oviduct and uterine epithelia in vitro

The composition of fluids within the bovine oviduct and uterine lumen, important in fertilisation and early embryonic development, is ultimately determined by the transport properties of the epithelial cells which line the lumen. A preparation has therefore been devised to study the role of these cells in oviduct and uterine fluid formation. Pure preparations of epithelial cells, as judged immunocytochemically, were isolated by enzyme digestion, and grown on collagen filters in primary culture. The cells re-establish intercellular junctions to form a confluent epithelial layer. Serial samples from the apical and basal media were analysed for K+, Na+, Ca2+, glucose and lactate. Bovine oviduct epithelial cells maintained gradients of K+ and Ca2+ (apical > basal) for up to 14 days after confluence, while bovine uterine epithelial cells maintained apical > basal gradients of K+. Both types of epithelium exhibited a small transepithelial electrical potential difference and a higher uptake of glucose and production of lactate in the basal, as opposed to apical medium. There were no consistent differences in any of these parameters with the stage of the oestrous cycle at which the cells were removed. The data indicate that bovine oviduct and uterine epithelia may be isolated and grown as polarised layers in primary culture. The preparations will now enable the mechanisms underlying the secretion of ions and non-electrolytes to be determined.

Effects of nucleosides and a nucleotide mixture on gut mucosal barrier function on parenteral nutrition in rats

BACKGROUND

We have previously reported that the addition of nucleosides and a nucleotide mixture (OG-VI) to total parenteral nutrition (TPN) prevents TPN-induced intestinal mucosal atrophy and results in increased intestinal cathepsin activities under TPN. The aims of the present study are to examine the effects of OG-VI on mucosal barrier functions and to clarify whether the intestinal cathepsins can be used as a new marker of mucosal barrier function.

METHODS

Male Wistar rats were divided into three groups: FED (food ad libitum), TPN (a standard TPN solution), and OG (OG-VI in addition to the TPN solution). Mucosal barrier functions were analyzed by the lactulose/mannitol (L/M) test, mucosal ZO-1 messenger RNA level, and electron micrographs in the gut. Intestinal cathepsin B, H, and L activities were also measured.

RESULTS

The L/M ratio for TPN was much higher than that for FED. Only lactulose excretion was significantly reduced by OG-VI. The intercellular spaces of the mucosal cells for TPN were significantly wider than those for FED, but were reduced in size by the addition of OG-VI. All cathepsin activities of the ileum were higher for TPN than for FED, whereas OG showed no increase.

CONCLUSION

OG-VI contributes to the improvement of gut mucosal barrier function. Intestinal cathepsin activities can be used as a new marker of mucosal barrier function.

Characterisation of the major intrinsic protein (MIP) from bovine lens fibre membranes by electron microscopy and hydrodynamics

The major intrinsic protein (MIP) from bovine lens fibre membranes has been purified from unstripped membranes using a single ion-exchange chromatography step (MonoS) in the non-ionic detergent octyl-beta-D-glucopyranoside (OG). SDS-PAGE has confirmed the purity of the preparation and thin-layer chromatographic analysis has shown that the protein is virtually lipid-free. To establish a stable and monodisperse protein sample, we exchanged OG with decyl-beta-D-maltopyranoside (DeM), another non-ionic detergent, by gel-filtration column chromatography. We conclude that the resulting protein/detergent complex is composed of four copies of MIP (a tetramer) and a detergent micelle. This conclusion is based on: (1) measurement of the weight-average molecular mass (Mw,app) of the protein moiety in the protein/detergent complex by sedimentation equilibrium; (2) measurement of the apparent molecular mass of the complexes formed by MIP in OG, in DeM, in dodecyl-beta-D-maltopyranoside (DoM) and in sodium dodecylsulphate (SDS) by gel filtration; (3) measurement of the apparent molecular mass of pure detergent micelles; (4) measurement of the predicted change in the molecular mass of the MIP/DeM complex after partial enzymatic proteolysis; and (5) measurement of the size and shape of the MIP/detergent complex by electron microscopy and single-particle analysis. Therefore, the tetragonal arrangement of MIP observed in both plasma membranes and junctional membranes in lens fibre cells is maintained in solution with non-ionic detergents.

A 155-kDa undercoat-constitutive protein of cell-to-cell adherens junctions

A fraction enriched in cell-to-cell adherens and tight junctions was isolated from the chick liver, and the undercoat-constitutive proteins were extracted from this isolated junctional fraction. Monoclonal antibodies (mAbs) were then obtained by injecting this extract into rats, and five antigens were identified to be concentrated in the isolated junctional fraction. We have characterized one mAb (E14 mAb) and its antigen (E14). By immunoblotting of the isolated junctional fraction the E14 mAb reacted strongly with a single band of approximately 155 kDa, and E14 was highly concentrated in the isolated junctional fraction. Immunofluorescence microscopy revealed that the E14 mAb exclusively stained the junctional complex region of the liver, renal epithelial cells, and the cell-cell border of endothelial cells in various tissues. The intercalated disc of the heart was also significantly stained. However, the E14 signal was hardly detected from intestinal epithelial cells. By immunoelectron microscopy using renal epithelial cells, E14 was mainly detected in the fibrous structures associated with the cell-to-cell adherens junction. We conclude that E14 is a novel undercoat-constitutive protein found in certain types of cell-to-cell adherens junctions.

Expression of CD44 isoforms is increased in the airway epithelium of asthmatic subjects

Since shedding of columnar, but not basal, epithelial cells is common in asthma, cell adhesion molecules such as CD44, which are differentially expressed on these cell types, are likely to be important in this disease. In bronchial epithelium of asthmatic and nonasthmatic subjects, CD44 isoforms have been localized by light- and electron-microscopic immunocytochemistry. Immunoreactivity for total CD44 (mAb Hermes-3/mAb 25.32) and for isoforms containing CD44v9 (mAb 11.24), CD44v6 (mAb 11.9), and CD44v4 (mAb 11.10) have been compared. In nonasthmatic samples, CD44s and CD44v9 were seen on basal but not columnar epithelial cells. Weak CD44v6 immunoreactivity was found infrequently in the bronchus, whereas CD44v4 immunoreactivity was absent. This indicates the presence of a distinct population of basal cells that express CD44. No CD44 was detected in areas of close cell-cell or cell-matrix contact, thus precluding the involvement of CD44 in stable adhesion in these areas. CD44 immunoreactivity was locally increased in areas showing morphologic damage to the epithelium. In epithelium from asthmatic subjects, the mean level of CD44 immunoreactivity on basal-cell membranes was doubled (4.3 versus 2.0 gold particles/microns membrane) as compared with nonasthmatic subjects. Increased expression of CD44 in asthmatic subjects, suggests that it has a significant role in the pathobiology of this disease, whereas the restricted distribution of this increase supports an association with repair rather than with inflammatory processes.

Calreticulin modulates cell adhesiveness via regulation of vinculin expression

Calreticulin is an ubiquitous and highly conserved high capacity Ca(2+)-binding protein that plays a major role in Ca2+ storage within the lumen of the ER. Here, using L fibroblast cell lines expressing different levels of calreticulin, we show that calreticulin plays a role in the control of cell adhesiveness via regulation of expression of vinculin, a cytoskeletal protein essential for cell-substratum and cell-cell attachments. Both vinculin protein and mRNA levels are increased in cells overexpressing calreticulin and are downregulated in cells expressing reduced level of calreticulin. Abundance of actin, talin, alpha 5 and beta 1 integrins, pp125 focal adhesion kinase, and alpha-catenin is not affected by the differential calreticulin expression. Overexpression of calreticulin increases both cell-substratum and cell-cell adhesiveness of L fibroblasts that, most surprisingly, establish vinculin-rich cell-cell junctions. Upregulation of calreticulin also affects adhesion-dependent phenomena such as cell motility (which decreases) and cell spreading (which increases). Downregulation of calreticulin brings about inverse effects. Cell adhesiveness is Ca2+ regulated. The level of calreticulin expression, however, has no effect on either the resting cytoplasmic Ca2+ concentration or the magnitude of FGF-induced Ca2+ transients. Calreticulin, however, participates in Ca2+ homeostasis as its level of expression affects cell viability at low concentrations of extracellular Ca2+. Consequently, we infer that it is not the Ca2+ storage function of calreticulin that affects cell adhesiveness. Neither endogenous calreticulin nor overexpressed green fluorescent protein-calreticulin construct can be detected outside of the ER. Since all of the adhesion-related effects of differential calreticulin expression can be explained by its regulation of vinculin expression, we conclude that it is the ER-resident calreticulin that affects cellular adhesiveness.

Healing of incisional wounds in the embryonic chick wing bud: characterization of the actin purse-string and demonstration of a requirement for Rho activation

Small skin wounds in the chick embryo do not heal by lamellipodial crawling of cells at the wound edge as a skin wound does in the adult, but rather by contraction of an actin purse-string that rapidly assembles in the front row of epidermal cells (Martin, P., and J. Lewis. 1992. Nature (Lond.). 360:179-183). To observe the early time course of actin purse-string assembly and to characterize other cytoskeletal components of the contractile machinery, we have followed the healing of incisional or slash wounds on the dorsum of the chick wing; these wounds take only seconds to create and heal within approximately 6 h. Healing of the epithelium depends on a combination of purse-string contraction and zipper-like closure of the gap between the cut edges of the epithelium. Confocal laser scanning microscope studies show that actin initially aligns into a cable at the wound margin in the basal layer of the epidermis within approximately 2 min of wounding. Coincident with actin cable assembly, we see localization of cadherins into clusters at the wound margin, presumably marking the sites where segments of the cable in adjacent cells are linked via adherens junctions. A few minutes later we also see localization of myosin II at the wound margin, as expected if myosin is being recruited into the cable to generate a contractile force for wound healing. At the time of wounding, cells at the wound edge become transiently leaky, allowing us to load them with reagents that block the function of two small GTPases, Rho and Rac, which recently have been shown to play key roles in reorganiztion of the actin cytoskeleton in tissue-culture cells (Hall, A. 1994. Annu. Rev. Cell Biol. 10:31-54). Loading wound edge epidermal cells with C3 transferase, a bacterial exoenzyme that inactivates endogenous Rho, prevents assembly of an actin cable and causes a failure of healing. No such effects are seen with N17rac, a dominant inhibitory mutant Rac protein. These findings support the view that in this system the actin cable is required for healing-both the purse-string contraction and the zipping up-and that Rho is required for formation of the actin cable.

The use of two-dimensional gel electrophoresis in studies on the role of cytoskeletal plaque proteins as tumor suppressors

The morphology and functions of cells and tissues are determined, in a large part, by mechanical forces generated at cell-cell and cell-extracellular matrix (ECM) contacts. At these sites, transmembrane adhesion receptors of the integrin and cadherin families are linked, via their cytoplasmic domain, to the cytoskeleton by submembranal plaque proteins such as vinculin, alpha-actinin and the cell-cell junctional plaque proteins alpha- and beta-catenin and plakoglobin (or gamma-catenin). Recent studies have implicated this link of structural molecules between the outside and inside of the cell in signal transduction. We have shown that the expression of junctional plaque proteins is modulated during growth stimulation and differentiation, and is dramatically reduced in certain tumor cells. To study the functional significance of these changes in expression, we have used recombinant DNA technologies to overexpress or suppress the levels of junctional plaque proteins. In addition, we eliminated the expression of vinculin in embryonal stem (ES) cells and in the embryonal carcinoma F9 line by gene disruption employing homologous recombination. The results have indicated that moderate overexpression of cell-ECM plaque proteins results in reduced cell motility. In contrast, suppression of their expression, by antisense transfection, led to enhanced motility and conferred anchorage independent growth and tumorigenicity, upon injection into nude mice. These findings suggest that submembranal plaque proteins can act as effective tumor suppressors. In agreement with this notion, we found in several tumor cell lines diminished levels of junctional plaque proteins. Restoration of their level to that found in normal cells resulted in tumor suppression after their injection into experimental animals. Here we demonstrate the usefulness of the application of two dimensional (2-D) gel electrophoresis in these studies.

Localization of cranin (dystroglycan) at sites of cell-matrix and cell-cell contact: recruitment to focal adhesions is dependent upon extracellular ligands

We report that cranin (dystroglycan) can become recruited to focal adhesions of cultured rat REF 52 fibroblasts and human aortic smooth muscle cells. Within mature focal adhesions, cranin was present within the plaque region defined by beta 1 integrin, vinculin and phosphotyrosine staining, but occupied a larger domain corresponding to the terminal segments of stress fibers that was more precisely co-extensive with the cytoskeletal proteins alpha-actinin, utrophin and aciculin. When REF 52 fibroblasts were plated on different substrata in the absence of protein synthesis and secretion in serum-free medium, focal clusters of cranin readily formed within 2 hours on matrix proteins that bind cranin directly (laminin or agrin) which were maintained as the focal adhesions became mature. In contrast, cranin failed to become targeted to cell-substratum attachment sites, either at early or later times, when cells were plated on a variety of other substrata that elicit formation of focal adhesions but do not bind cranin directly (fibronectin, vitronectin, collagen type IV, or anti-beta 1 integrin antibody TS2/16). These data strongly suggest that targeting of cranin to focal adhesions was dependent upon the presence of an extracellular ligand capable of binding cranin directly. However, some cultured nonmuscle cell lines (e.g., human umbilical vein endothelial cells, NIH 3T3 and CHO cells) failed to localize cranin to focal adhesions, even when plated on laminin. Cranin was also enriched at cell-cell adherens-type junctions of human normal breast MCF-10 epithelial cells, and at growth cones of E17 rat hippocampal axons. That cranin can become targeted to sites of cell-cell and cell-substratum contact in diverse cell types supports the hypothesis that cranin may be involved in mediating or regulating cell adhesion. The absence of muscle-specific and synapse-specific proteins within fibroblasts and epithelial cells provides a different context for thinking about cranin (dystroglycan) that may aid in discerning general principles of its structure and function.

Targeted mutation of plakoglobin in mice reveals essential functions of desmosomes in the embryonic heart

Plakoglobin (gamma-catenin), a member of the armadillo family of proteins, is a constituent of the cytoplasmic plaque of desmosomes as well as of other adhering cell junctions, and is involved in anchorage of cytoskeletal filaments to specific cadherins. We have generated a null mutation of the plakoglobin gene in mice. Homozygous -/- mutant animals die between days 12-16 of embryogenesis due to defects in heart function. Often, heart ventricles burst and blood floods the pericard. This tissue instability correlates with the absence of desmosomes in heart, but not in epithelia organs. Instead, extended adherens junctions are formed in the heart, which contain desmosomal proteins, i.e., desmoplakin. Thus, plakoglobin is an essential component of myocardiac desmosomes and seems to play a crucial role in the sorting out of desmosomal and adherens junction components, and consequently in the architecture of intercalated discs and the stabilization of heart tissue.

Prolonged cytokine exposure causes a dynamic redistribution of endothelial cell adhesion molecules to intercellular junctions

After 4 hours of treatment with TNF, newly synthesized endothelial leukocyte adhesion molecule 1, vascular cell adhesion molecule 1, and intercellular adhesion molecule 1 molecules are diffusely expressed on the apical surface of cultured umbilical vein endothelial cells. Such cells maintain the epithelioid, cobblestone appearance of untreated endothelial cells and display cytoskeletal actin largely arranged in dense peripheral bands. After 24 to 72 hours of treatment with TNF, cells become elongated and rearrange their actin filaments into longitudinal stress fibers. At this time, intercellular adhesion molecule 1 and vascular cell adhesion molecule 1 remain elevated but redistribute to the cell junctions. Intercellular adhesion molecule 2, beta 1 integrins, and beta 3 integrins also redistribute to cell junctions in TNF-treated cultures. IFN-gamma produces morphologic changes similar to those induced by TNF but does not cause surface protein redistribution. Cells treated with TNF plus IFN-gamma become even more elongated and display TNF-like redistributions. We conclude that TNF activates a program of membrane protein redistribution, and we speculate that this dynamic redistribution of adhesion molecules to cell junctions may contribute to the recruitment of leukocytes to sites of inflammation.

Adhesive proteins at endothelial cell-to-cell junctions and leukocyte extravasation

Endothelial cell junctions are complex structures formed by transmembrane adhesive molecules linked to a network of cytoplasmic/cytoskeletal proteins. At least four different types of endothelial junctions have been described (tight junctions, gap junctions, adherence junctions and syndesmos or complexus adhaerentes). Leukocytes adhesion to endothelial cells is frequently followed by their extravasation. The mechanisms which regulate the passage of leukocytes through endothelial clefts remain to be clarified. Many indirect data suggest that leukocytes might transfer signals to endothelial cells both through the release of active agents and adhesion to the endothelial cell surface. These signals could induce the disorganization of interendothelial junctions and facilitate leukocyte transmigration.

Neuronal nitric oxide synthase and dystrophin-deficient muscular dystrophy

Neuronal nitric oxide synthase (nNOS) in fast-twitch skeletal muscle fibers is primarily particulate in contrast to its greater solubility in brain. Immunohistochemistry shows nNOS localized to the sarcolemma, with enrichment at force transmitting sites, the myotendinous junctions, and costameres. Because this distribution is similar to dystrophin, we determined if nNOS expression was affected by the loss of dystrophin. Significant nNOS immunoreactivity and enzyme activity was absent in skeletal muscle tissues from patients with Duchenne muscular dystrophy. Similarly, in dystrophin-deficient skeletal muscles from mdx mice both soluble and particulate nNOS was greatly reduced compared with C57 control mice. nNOS mRNA was also reduced in mdx muscle in contrast to mRNA levels for a dystrophin binding protein, alpha 1-syntrophin. nNOS levels increased dramatically from 2 to 52 weeks of age in C57 skeletal muscle, which may indicate a physiological role for NO in aging-related processes. Biochemical purification readily dissociates nNOS from the dystrophin-glycoprotein complex. Thus, nNOS is not an integral component of the dystrophin-glycoprotein complex and is not simply another dystrophin-associated protein since the expression of both nNOS mRNA and protein is affected by dystrophin expression.

Neuron-glia interrelations during migration of Purkinje cells in the mouse embryonic cerebellum

The interrelations between migratory Purkinje cells and radial glial processes were examined immunohistochemically and electron-microscopically in the fetal mouse cerebellum. Migratory Purkinje cells identified immunochemically with anti-spot 35 antibody were apposed to the presumed radial glial processes. Putative immature migratory Purkinje cells were apposed to the radial glial processes identified immunochemically with monoclonal antibody 1D11 or anti-tenascin antibody. Junctional specializations related to cell adhesion were observed at the sites of contact between the immature neurons and glial processes. Furthermore, coated vesicles and coated pits were noted in the contact region. These findings support the concept of contact guidance of migratory Purkinje cells by radial glial processes through cell adhesion, and also suggest the trophic interactions occurring at the region of contact.

Are there functional gap junctions or junctional hemichannels in macrophages?

The existence of functional gap junctions in migratory cells of the immune system is a controversial issue. In this report, we have focused on one particular cell type, namely the macrophages, because connexin-43, a protein that forms gap junctions, has been described in peritoneal macrophages and a macrophage cell line (J774), by Northern and Western blot analysis. To test whether these cell types expressed functional gap junctions, we assayed dye coupling by intracellular injection of Lucifer Yellow. We observed that nonstimulated macrophages are not coupled among themselves and did not form functional gap junctions with an epithelial cell line, which expresses functional gap junctions formed by connexin-43. Dye coupling was also not detected between macrophages previously activated by lipopolysaccharide or interferon-gamma. We further examined the presence of functional coupling using the more sensitive technique of dual whole cell patch-clamp, and again, did not find electrical coupling between macrophages, consistent with the dye microinjection data. We also examined the possible presence of hemigap junction channels activated by extracellular adenosine triphosphate (ATP) using a dye uptake assay and the whole cell patch-clamp technique. Conditions expected to close gap junction hemichannels (exposure to octanol and low intracellular pH) did not decrease ATP-induced Lucifer Yellow uptake, whereas conditions expected to increase hemichannel opening either did not affect ATP permeabilization (dibutyryl adenosine monophosphate) or decreased it (zero extracellular CA+2). Finally, in experiments using resident macrophages derived from conexin-43 knockout mice, we observed ATP induced dye uptake. Our experimental data thus indicate that macrophages in vitro do not form functional gap junctions and that the permeability pathway activated by extracellular ATP is not formed by a hemigap junction channel.