DRG1 Maintains Intestinal Epithelial Cell Junctions and Barrier Function by Regulating RAC1 Activity in Necrotizing Enterocolitis

BACKGROUND

An immature intestine is a high-risk factor for necrotizing enterocolitis (NEC), which is a serious intestinal disease in newborns. The regulation of developmentally regulated GTP-binding protein 1 (DRG1) during organ development suggests a potential role of DRG1 in the maturation process of the intestine.

AIM

To illustrate the function of DRG1 during the pathogenesis of NEC.

METHODS

DRG1 expression in the intestine was measured using immunohistochemistry and q-PCR. Immunoprecipitation coupled with mass spectrometry was used to identify the interacting proteins of DRG1. The biological functions of the potential interactors were annotated with the Database for Annotation, Visualization and Integrated Discovery. Caco2 and FHs74Int cells with stable DRG1 silencing or overexpression were used to investigate the influence of DRG1 on cell junctions and intestinal barrier permeability and to elucidate the downstream mechanism.

RESULTS

DRG1 was constitutively expressed during the intestinal maturation process but significantly decreased in the ileum in the context of NEC. Protein interaction analysis revealed that DRG1 was closely correlated with cell junctions. DRG1 deficiency destabilized the E-cadherin and occludin proteins near the cell membrane and increased the permeability of the epithelial cell monolayer, while DRG1 overexpression prevented lipopolysaccharide-induced disruption of E-cadherin and occludin expression and cell monolayer integrity. Further investigation suggested that DRG1 maintained cell junctions, especially adherens junctions, by regulating RAC1 activity, and RAC1 inhibition with NSC23766 attenuated intestinal injury and led to improved barrier integrity in experimental NEC.

CONCLUSIONS

Our findings illustrate the mechanism underlying the effect of DRG1 deficiency on epithelial cell permeability regulation and provide evidence supporting the application of RAC1 inhibitors for protection against NEC.

ROCK2 primes the endothelium for vascular hyperpermeability responses by raising baseline junctional tension

Rho kinase mediates the effects of inflammatory permeability factors by increasing actomyosin-generated traction forces on endothelial adherens junctions, resulting in disassembly of intercellular junctions and increased vascular leakage. In vitro, this is accompanied by the Rho kinase-driven formation of prominent radial F-actin fibers, but the in vivo relevance of those F-actin fibers has been debated, suggesting other Rho kinase-mediated events to occur in vascular leak. Here, we delineated the contributions of the highly homologous isoforms of Rho kinase (ROCK1 and ROCK2) to vascular hyperpermeability responses. We show that ROCK2, rather than ROCK1 is the critical Rho kinase for regulation of thrombin receptor-mediated vascular permeability. Novel traction force mapping in endothelial monolayers, however, shows that ROCK2 is not required for the thrombin-induced force enhancements. Rather, ROCK2 is pivotal to baseline junctional tension as a novel mechanism by which Rho kinase primes the endothelium for hyperpermeability responses, independent from subsequent ROCK1-mediated contractile stress-fiber formation during the late phase of the permeability response.

Inhibition of Rho and Rac geranylgeranylation by atorvastatin is critical for preservation of endothelial junction integrity

Background

Small GTPases (guanosine triphosphate, GTP) are involved in many critical cellular processes, including inflammation, proliferation, and migration. GTP loading and isoprenylation are two important post-translational modifications of small GTPases, and are critical for their normal function. In this study, we investigated the role of post-translational modifications of small GTPases in regulating endothelial cell inflammatory responses and junctional integrity.

Methods and Results

Confluent human umbilical vein endothelial cell (HUVECs ) treated with atorvastatin demonstrated significantly decreased lipopolysaccharide (LPS)-mediated IL-6 and IL-8 generation. The inhibitory effect of atorvastatin (Atorva) was attenuated by co-treatment with 100 µM mevalonate (MVA) or 10 µM geranylgeranyl pyrophosphate (GGPP), but not by 10 µM farnesyl pyrophosphate (FPP). Atorvastatin treatment of HUVECs produced a time-dependent increase in GTP loading of all Rho GTPases, and induced the translocation of small Rho GTPases from the cellular membrane to the cytosol, which was reversed by 100 µM MVA and 10 µM GGPP, but not by 10 µM FPP. Atorvastatin significantly attenuated thrombin-induced HUVECs permeability, increased VE-cadherin targeting to cell junctions, and preserved junction integrity. These effects were partially reversed by GGPP but not by FPP, indicating that geranylgeranylation of small GTPases plays a major role in regulating endothelial junction integrity. Silencing of small GTPases showed that Rho and Rac, but not Cdc42, play central role in HUVECs junction integrity.

Conclusions

In conclusion, our studies show that post-translational modification of small GTPases plays a vital role in regulating endothelial inflammatory response and endothelial junction integrity. Atorvastatin increased GTP loading and inhibited isoprenylation of small GTPases, accompanied by reduced inflammatory response and preserved cellular junction integrity.

Rho/Rho-associated kinase-II signaling mediates disassembly of epithelial apical junctions

Apical junctional complex (AJC) plays a vital role in regulation of epithelial barrier function. Disassembly of the AJC is observed in diverse physiological and pathological states; however, mechanisms governing this process are not well understood. We previously reported that the AJC disassembly is driven by the formation of apical contractile acto-myosin rings. In the present study, we analyzed the signaling pathways regulating acto-myosin-dependent disruption of AJC by using a model of extracellular calcium depletion. Pharmacological inhibition analysis revealed a critical role of Rho-associated kinase (ROCK) in AJC disassembly in calcium-depleted epithelial cells. Furthermore, small interfering RNA (siRNA)-mediated knockdown of ROCK-II, but not ROCK-I, attenuated the disruption of the AJC. Interestingly, AJC disassembly was not dependent on myosin light chain kinase and myosin phosphatase. Calcium depletion resulted in activation of Rho GTPase and transient colocalization of Rho with internalized AJC proteins. Pharmacological inhibition of Rho prevented AJC disassembly. Additionally, Rho guanine nucleotide exchange factor (GEF)-H1 translocated to contractile F-actin rings after calcium depletion, and siRNA-mediated depletion of GEF-H1 inhibited AJC disassembly. Thus, our findings demonstrate a central role of the GEF-H1/Rho/ROCK-II signaling pathway in the disassembly of AJC in epithelial cells.

A pump-independent function of the Na,K-ATPase is required for epithelial junction function and tracheal tube-size control

The heterodimeric Na,K-ATPase has been implicated in vertebrate and invertebrate epithelial cell junctions, morphogenesis and oncogenesis, but the mechanisms involved are unclear. We previously showed that the Drosophila Na,K-ATPase is required for septate junction (SJ) formation and that of the three beta-subunit loci, only Nrv2 isoforms support epithelial SJ barrier function and tracheal tube-size control. Here we show that Nrv1 is endogenously co-expressed with Nrv2 in the epidermis and tracheal system, but Nrv1 has a basolateral localization and appears to be excluded from the Nrv2-containing SJs. When the normally neuronal Nrv3 is expressed in epithelial cells, it does not associate with SJs. Thus, the beta-subunit is a key determinant of Na,K-ATPase subcellular localization as well as function. However, localization of the Na,K-ATPase to SJs is not sufficient for junctional activity because although several Nrv2/Nrv3 chimeric beta-subunits localize to SJs, only those containing the extracellular domain of Nrv2 have junctional activity. Junctional activity is also specific to different alpha-subunit isoforms, with only some isoforms from the major alpha-subunit locus being able to provide full barrier function and produce normal tracheal tubes. Importantly, mutations predicted to inactivate ATPalpha catalytic function do not compromise junctional activity, demonstrating that the Drosophila Na,K-ATPase has an ion-pump-independent role in junction formation and tracheal morphogenesis. These results define new functions for the intensively studied Na,K-ATPase. Strikingly, the rat alpha1 isoform has full junctional activity and can rescue Atpalpha-null mutants to viability, suggesting that the Na,K-ATPase has an evolutionarily conserved role in junction formation and function.

Conversion of midbodies into germ cell intercellular bridges

Whereas somatic cell cytokinesis resolves with abscission of the midbody, resulting in independent daughter cells, germ cell cytokinesis concludes with the formation of a stable intercellular bridge interconnecting daughter cells in a syncytium. While many proteins essential for abscission have been discovered, until recently, no proteins essential for mammalian germ cell intercellular bridge formation have been identified. Using TEX14 as a marker for the germ cell intercellular bridge, we show that TEX14 co-localizes with the centralspindlin complex, mitotic kinesin-like protein 1 (MKLP1) and male germ cell Rac GTPase-activating protein (MgcRacGAP) and converts these midbody matrix proteins into stable intercellular bridge components. In contrast, septins (SEPT) 2, 7 and 9 are transitional proteins in the newly forming bridge. In cultured somatic cells, TEX14 can localize to the midbody in the absence of other germ cell-specific factors, suggesting that TEX14 serves to bridge the somatic cytokinesis machinery to other germ cell proteins to form a stable intercellular bridge essential for male reproduction.

Matrix-specific p21-activated kinase activation regulates vascular permeability in atherogenesis

Elevated permeability of the endothelium is thought to be crucial in atherogenesis because it allows circulating lipoproteins to access subendothelial monocytes. Both local hemodynamics and cytokines may govern endothelial permeability in atherosclerotic plaque. We recently found that p21-activated kinase (PAK) regulates endothelial permeability. We now report that onset of fluid flow, atherogenic flow profiles, oxidized LDL, and proatherosclerotic cytokines all stimulate PAK phosphorylation and recruitment to cell-cell junctions. Activation of PAK is higher in cells plated on fibronectin (FN) compared to basement membrane proteins in all cases. In vivo, PAK is activated in atherosclerosis-prone regions of arteries and correlates with FN in the subendothelium. Inhibiting PAK in vivo reduces permeability in atherosclerosis-prone regions. Matrix-specific PAK activation therefore mediates elevated vascular permeability in atherogenesis.

Adjudin-mediated junction restructuring in the seminiferous epithelium leads to displacement of soluble guanylate cyclase from adherens junctions

A plethora of evidence supports the role of cyclic nucleotides in junction restructuring. For instance, studies have shown cGMP to be a key regulator of junction assembly and disassembly in different in vitro and in vivo systems. In this study, we examine the role of soluble guanylate cyclase (sGC) in junction restructuring in the seminiferous epithelium of the rat testis. First, the interaction of soluble guanylate cyclase beta1 (sGCbeta1; sGC is a heterodimer comprised of an alpha and a beta subunit) with proteins that constitute adherens and tight junctions in the testis was demonstrated. By immunoprecipitation, sGCbeta1 was found to associate with occludin, JAM-A, and ZO-1, as well as with cadherin, catenin, nectin, afadin, ponsin, and espin, suggestive of its role in cell junction dynamics. These results were corroborated in part by immunohistochemistry experiments, which revealed that the localization of sGCbeta1 was largely restricted to the site of the apical and basal ectoplasmic specialization. Next, the role of sGC in junction dynamics was addressed by using an in vivo model of junction restructuring. Administration of Adjudin--a chemical entity known to specifically perturb adhesion between Sertoli and germ cells (i.e., round and elongate(ing) spermatids and most spermatocytes)--resulted in a approximately 1.5-fold increase in sGCbeta1, coinciding with the loss of germ cells from the epithelium. More importantly, the ability of sGCbeta1 to associate with cadherin increased approximately three-fold during Adjudin-mediated restructuring of Sertoli-germ cell junctions, whereas its interaction with tight junction proteins (i.e., occludin and ZO-1) decreased. Taken collectively, these results suggest that sGC participates in the remodeling of cell junctions during spermatogenesis.

Septate-junction-dependent luminal deposition of chitin deacetylases restricts tube elongation in the Drosophila trachea

The function of tubular epithelial organs like the kidney and lung is critically dependent on the length and diameter of their constituting branches. Genetic analysis of tube size control during Drosophila tracheal development has revealed that epithelial septate junction (SJ) components and the dynamic chitinous luminal matrix coordinate tube growth. However, the underlying molecular mechanisms controlling tube expansion so far remained elusive. Here, we present the analysis of two luminal chitin binding proteins with predicted polysaccharide deacetylase activities (ChLDs). ChLDs are required to assemble the cable-like extracellular matrix (ECM) and restrict tracheal tube elongation. Overexpression of native, but not of mutated, ChLD versions also interferes with the structural integrity of the intraluminal ECM and causes aberrant tube elongation. Whereas ChLD mutants have normal SJ structure and function, the luminal deposition of the ChLD requires intact cellular SJs. This identifies a new molecular function for SJs in the apical secretion of ChLD and positions ChLD downstream of the SJs in tube length control. The deposition of the chitin luminal matrix first promotes and coordinates radial tube expansion. We propose that the subsequent structural modification of chitin by chitin binding deacetylases selectively instructs the termination of tube elongation to the underlying epithelium.

Localization of alkaline phosphatase and proteins related to intercellular junctions in primary cultures of fetal rat hepatocytes

The localization of alkaline phosphatase (ALP) and four proteins related to intercellular junctions in primary cultures of fetal rat hepatocytes was immunocytochemically investigated using fluorescence-labeled antibodies and confocal laser microscopy in order to determine whether the formation of intercellular junctions at the borders between adjacent rat hepatocytes becomes the trigger of translocation of ALP from the cytoplasm to the plasma membrane. Dexamethasone (DEX) which was supplemented in the base medium promoted the translocation of ALP from the cytoplasm to the plasma membrane surrounding bile canaliculus-like intercellular spaces and the appearance of connexin-32 at cell borders between adjacent fetal hepatocytes. E-cadherin, occludin and ZO-1 were localized at the cell borders between adjacent fetal hepatocytes irrespective of the presence of DEX. Occludin and ZO-1 were further localized along the plasma membrane surrounding bile canaliculus-like intercellular spaces formed by DEX. The present study indicates that the formation of adherens and tight junctions between adjacent rat hepatocytes does not become the trigger of ALP translocation from the cytoplasm to the plasma membrane, although we cannot be certain of whether the formation of gap junctions between adjacent rat hepatocytes triggers ALP translocation.

MEMBRANE-ASSOCIATED GUANYLATE KINASES REGULATE ADHESION AND PLASTICITY AT CELL JUNCTIONS

Tissue development, differentiation, and physiology require specialized cellular adhesion and signal transduction at sites of cell-cell contact. Scaffolding proteins that tether adhesion molecules, receptors, and intracellular signaling enzymes organize macromolecular protein complexes at cellular junctions to integrate these functions. One family of such scaffolding proteins is the large group of membrane-associated guanylate kinases (MAGUKs). Genetic studies have highlighted critical roles for MAGUK proteins in the development and physiology of numerous tissues from a variety of metazoan organisms. Mutation of Drosophila discs large (dlg) disrupts epithelial septate junctions and causes overgrowth of imaginal discs. Similarly, mutation of lin-2, a related MAGUK in Caenorhabditis elegans, blocks vulval development, and mutation of the postsynaptic density protein PSD-95 impairs synaptic plasticity in mammalian brain. These diverse roles are explained by recent biochemical and structural analyses of MAGUKs, which demonstrate their capacity to assemble well--efined--yet adaptable--protein complexes at cellular junctions.

Colocalization of eNOS and the catalytic subunit of PKA in endothelial cell junctions: a clue for regulated NO production

Localization and coordinate phosphorylation/dephosphorylation of endothelial nitric oxide synthase (eNOS) are critical determinants for the basal and stimulated production of nitric oxide. Several phosphorylation sites in eNOS have been identified as targets of the cAMP-dependent protein kinase A (PKA). Basal eNOS activity is also regulated by interaction with caveolin-1, the major coat protein of caveolae. In the present study we have examined in rat aorta endothelium the subcellular steady-state distribution of eNOS, the catalytic subunit of PKA (PKA-c), and caveolin-1. Basal eNOS expression was found in two distinct locations, the endothelial cell surface and the Golgi complex. Cell surface eNOS was equally distributed over caveolar and non-caveolar membranes but was 2.5-fold enriched on luminal lamellipodia located at endothelial cell contacts. PKA-c colocalized with eNOS in the lamellipodia, whereas caveolin-1 was absent from these membrane domains. PKA-c was also found associated with cell surface caveolae and with tubulovesicular membranes of Golgi complex and endosomes. The topological proximity of eNOS with the catalytic subunit of PKA in restricted intracellular locations may provide mechanisms for differential PKA-mediated eNOS regulation.

Galpha12 regulates epithelial cell junctions through Src tyrosine kinases

Regulation and assembly of the epithelial cell junctional complex involve multiple signaling mechanisms, including heterotrimeric G proteins. Recently, we demonstrated that Galpha12 binds to the tight junction scaffolding protein ZO-1 through the SH3 domain and that activated Galpha12 increases paracellular permeability in Madin-Darby canine kidney (MDCK) cells (Meyer et al. J Biol Chem 277: 24855-24858, 2002). In the present studies, we explore the effects of Galpha12 expression on tight and adherens junction proteins and examine downstream signaling pathways. By confocal microscopy, we detect disrupted tight and adherens junction proteins with increased actin stress fibers in constitutively active Galpha12 (QLalpha12)-expressing MDCK cells. The normal distribution of ZO-1 and Na-K-ATPase was altered in QLalpha12-expressing MDCK cells, consistent with loss of polarity. We found that the tyrosine kinase inhibitor genistein and the Src-specific inhibitor PP-2 reversibly abrogated the QLalpha12 phenotype on the junctional complex. Junctional protein localization was preserved in PP-2- or genistein-treated QLalpha12-expressing cells, and the increase in paracellular permeability as measured by transepithelial resistance and [3H]mannitol flux was prevented by the inhibitors. Src activity was increased in QLalpha12-expressing MDCK cells as assessed by Src autophosphorylation, and beta-catenin tyrosine phosphorylation was also increased, although there was no detectable increase in Rho activity. Taken together, these results indicate that Galpha12 regulates MDCK cell junctions, in part through Src tyrosine kinase pathways.

Neuroglian, Gliotactin, and the Na+/K+ ATPase are essential for septate junction function in Drosophila

One essential function of epithelia is to form a barrier between the apical and basolateral surfaces of the epithelium. In vertebrate epithelia, the tight junction is the primary barrier to paracellular flow across epithelia, whereas in invertebrate epithelia, the septate junction (SJ) provides this function. In this study, we identify new proteins that are required for a functional paracellular barrier in Drosophila. In addition to the previously known components Coracle (COR) and Neurexin (NRX), we show that four other proteins, Gliotactin, Neuroglian (NRG), and both the alpha and beta subunits of the Na+/K+ ATPase, are required for formation of the paracellular barrier. In contrast to previous reports, we demonstrate that the Na pump is not localized basolaterally in epithelial cells, but instead is concentrated at the SJ. Data from immunoprecipitation and somatic mosaic studies suggest that COR, NRX, NRG, and the Na+/K+ ATPase form an interdependent complex. Furthermore, the observation that NRG, a Drosophila homologue of vertebrate neurofascin, is an SJ component is consistent with the notion that the invertebrate SJ is homologous to the vertebrate paranodal SJ. These findings have implications not only for invertebrate epithelia and barrier functions, but also for understanding of neuron-glial interactions in the mammalian nervous system.

Platelet-activating factor induces lung inflammation and leak in rats: hydrogen peroxide production along neutrophil-lung endothelial cell interfaces

Interleukin-1 (IL-1) levels and neutrophils are increased in the lung-lavage fluid of patients with acute lung injury (ALI), and instilling IL-1 intratracheally into rats causes rapid lung-neutrophil influx and neutrophil-dependent lung leakage. IL-1 insufflation also increases platelet-activating factor (PAF) activity in rat lung, and PAF is increased in the lung-lavage fluid of ALI patients. To assess the direct effects of PAF on the lung, we administered PAF intratracheally in rats. We found that rats given PAF (5 microg) intratracheally had increased lung nuclear factor-kappaB activation, myeloperoxidase activity, numbers of lavage neutrophils, lavage neutrophil nitroblue tetrazolium reduction, and leakage compared with sham-treated rats administered saline solution intratracheally. Electron microscopic examination also indicated that lungs from rats given PAF intratracheally had increased neutrophil infiltration, cell damage, and neutrophil-endothelial cell interface cerium chloride staining - a marker of hydrogen peroxide production - compared with sham-treated rats. Simultaneous treatment with a PAF receptor-antagonist, WEB 2086, decreased the aforementioned changes observed after intratracheal administration of PAF.

Targets for pharmacological intervention of endothelial hyperpermeability and barrier function

Many diseases share the common feature of vascular leakage, and endothelial barrier dysfunction is often the underlying cause. The subsequent stages of endothelial barrier dysfunction contribute to endothelial hyperpermeability. Vasoactive agents induce loss of junctional integrity, a process that involves actin-myosin interaction. Subsequently, the interaction of leukocytes amplifies leakage by the leukocyte-derived mediators. The processes mainly occur at the postcapillary venules. The whole microvascular bed, including the capillaries, becomes involved in vascular leakage by the induction of angiogenesis. Plasma leakage results from gaps between endothelial cells as well as by the induction of transcellular transport pathways. Several mechanisms can improve endothelial barrier function, depending on the tissue affected and the cause of hyperpermeability. They include blockade of specific receptors and elevation of cyclic AMP (cAMP) by agents such as beta(2)-adrenergic agents. However, current therapies based on these principles often fail. Recent research has identified several new promising targets for pharmacological therapy. Endogenous compounds were also found with barrier-improving characteristics. Important insights were obtained in the different pathways involved in barrier dysfunction. Such insights regard the regulation of endothelial contraction and endothelial junction integrity: inhibitors of RhoA activation and Rho kinase represent a potentially valuable group of agents with endothelial hyperpermeability reducing properties, and strategies to target vascular endothelial growth factor (VEGF)-mediated edema are under current investigation. In clinical practice, not only tools to improve an impaired endothelial barrier function are necessary. Sometimes, a controlled, temporal, and local increase in permeability can also be desired, for example, with the aim to enhance drug delivery. Therefore, vessel leakiness is also being exploited to enable tissue access of liposomes, viral vectors, and other therapeutic agents that do not readily cross healthy endothelium. This review discusses strategies for targeting signaling molecules in therapies for diseases involving altered endothelial permeability.

Functional role of alpha-actinin, PI 3-kinase and MEK1/2 in insulin-like growth factor I receptor kinase regulated motility of human breast carcinoma cells

Within epithelial tissue, cells are held together by specialized lateral junctions. At particular stages of development and in pathological processes such as metastasis, cells break down the intercellular junctions, separate from the epithelial sheet and migrate individually. Despite the importance of these processes, little is understood about the regulatory mechanisms of active cell separation. In view of the effects of insulin-like growth factor I (IGF-I) on mammary gland development and cancer, we developed a model using MCF-7 human breast cancer cells in which the process of cell separation can be induced by IGF-I. The separation was enhanced in MCF-7 cells overexpressing the IGF-IR and blocked in the cells expressing a dead-kinase mutant of this receptor. Activation of the IGF-IR resulted in a rapid formation of motile actin microspikes at the regions of cell-cell contacts, disorganization of mature adherens junctions and the onset of cell migration. In cell separation, the signaling between the IGF-IR kinase and actin required phosphatidylinositol 3 (PI 3)-kinase-generated phospholipids but not MAP kinases and was mediated by alpha-actinin. The activity of MEK1/2 kinases was needed for consecutive cell migration. This work also defined a new function for alpha-actinin. Upon IGF-IR activation, green fluorescence protein (GFP)-labeled alpha-actinin concentrated at the base of actin microspikes. Deletion of the N-terminal actin-binding domain of alpha-actinin prevented this redistribution, indicating that this domain is necessary. Detection of the C-terminal tail of alpha-actinin reduced the number of microspikes, showing that alpha-actinin has a role in the development of microspikes and is not passively reorganized with filamentous actin. We suggest that the signaling pathway from the IGF-IR kinase through the PI-3 kinase to alpha-actinin participates in the rapid organization of actin into microspikes at the cell-cell junctions and leads to active cell separation, whereas signaling through ERK1/2 MAP kinases controls cell migration following cell separation.

aPKC kinase activity is required for the asymmetric differentiation of the premature junctional complex during epithelial cell polarization

We have previously shown that aPKC interacts with cell polarity proteins PAR-3 and PAR-6 and plays an indispensable role in cell polarization in the C. elegans one-cell embryo as well as in mammalian epithelial cells. Here, to clarify the molecular basis underlying this aPKC function in mammalian epithelial cells, we analyzed the localization of aPKC and PAR-3 during the cell repolarization process accompanied by wound healing of MTD1-A epithelial cells. Immunofluorescence analysis revealed that PAR-3 and aPKClambda translocate to cell-cell contact regions later than the formation of the primordial spot-like adherens junctions (AJs) containing E-cadherin and ZO-1. Comparison with three tight junction (TJ) membrane proteins, JAM, occludin and claudin-1, further indicates that aPKClambda is one of the last TJ components to be recruited. Consistently, the expression of a dominant-negative mutant of aPKClambda (aPKClambdakn) in wound healing cells does not inhibit the formation of the spot-like AJs; rather, it blocks their development into belt-like AJs. These persistent spot-like AJs in aPKClambda-expressing cells contain all TJ membrane proteins and PAR-3, indicating that aPKC kinase activity is not required for their translocation to these premature junctional complexes but is indispensable for their further differentiation into belt-like AJs and TJs. Cortical bundle formation is also blocked at the intermediate step where fine actin bundles emanating from premature cortical bundles link the persistent spot-like AJs at apical tips of columnar cells. These results suggest that aPKC contributes to the establishment of epithelial cell polarity by promoting the transition of fibroblastic junctional structures into epithelia-specific asymmetric ones.

Cell-cell signaling by direct contact increases cell proliferation via a PI3K-dependent signal

We report a novel mechanism of cellular growth control. Increasing the density of endothelial or smooth muscle cells in culture increased cell-cell contact and decreased cell spreading, leading to growth arrest. Using a new method to independently control cell-cell contact and cell spreading, we found that introducing cell-cell contact positively regulates proliferation, but that contact-mediated proliferation can be masked by changes in cell spreading: Round cells with many contacts proliferated less than spread cells with none. Physically blocking cell-cell contact or inhibiting PI3K signaling abrogated cell-cell induced proliferation, but inhibiting diffusible paracrine signaling did not. Thus, direct cell-cell contact induces proliferation in these cells.

Posterior midgut epithelial cells differ in their organization of the membrane skeleton from other drosophila epithelia

In epithelial cells, the various components of the membrane skeleton are segregated within specialized subregions of the plasma membrane, thus contributing to the development and stabilization of cell surface polarity. It has previously been shown that, in various Drosophila epithelia, the membrane skeleton components ankyrin and alphabeta-spectrin reside at the lateral surface, whereas alphabeta(H)-spectrin is restricted to the apical domain. By use of confocal immunofluorescence microscopy, the present study characterizes the membrane skeleton of epithelial cells in the posterior midgut, leading to a number of unexpected results. First, ankyrin and alphabeta-spectrin are not detected on the entire lateral surface but appear to be restricted to the apicolateral area, codistributing with fasciclin III at smooth septate junctions. The presumptive ankyrin-binding proteins neuroglian and Na(+),K(+)-ATPase, however, do not colocalize with ankyrin. Second, alphabeta(H)-spectrin is enriched at the apical domain but is also present in lower amounts on the entire lateral surface, colocalizing apicolaterally with ankyrin/alphabeta-spectrin. Finally, despite the absence of zonulae adherentes, F-actin, beta(H)-spectrin, and nonmuscle myosin-II are enriched in the midlateral region. Thus, the model established for the organization of the membrane skeleton in Drosophila epithelia does not hold for the posterior midgut, and there is quite some variability between the different epithelia with respect to the organization of the membrane skeleton.

The p38 mitogen-activated protein kinase mediates cytoskeletal remodeling in pulmonary microvascular endothelial cells upon intracellular adhesion molecule-1 ligation

Changes in the cytoskeleton of endothelial cells (ECs) play important roles in mediating neutrophil migration during inflammation. Previous studies demonstrated that neutrophil adherence to TNF-alpha-treated pulmonary microvascular ECs induced cytoskeletal remodeling in ECs that required ICAM-1 ligation and oxidant production and was mimicked by cross-linking ICAM-1. In this study, we examined the role of ICAM-1-induced signaling pathways in mediating actin cytoskeletal remodeling. Cross-linking ICAM-1 induced alterations in ICAM-1 distribution, as well as the filamentous actin rearrangements and stiffening of ECs shown previously. ICAM-1 cross-linking induced phosphorylation of the p38 mitogen-activated protein kinase (MAPK) that was inhibited by allopurinol and also induced an increase in the activity of the p38 MAPK that was inhibited by SB203580. However, SB203580 had no effect on oxidant production in ECs or ICAM-1 clustering. ICAM-1 cross-linking also induced phosphorylation of heat shock protein 27, an actin-binding protein that may be involved in filamentous actin polymerization. The time course of heat shock protein 27 phosphorylation paralleled that of p38 MAPK phosphorylation and was completely inhibited by SB203580. In addition, SB203580 blocked the EC stiffening response induced by either neutrophil adherence or ICAM-1 cross-linking. Moreover, pretreatment of ECs with SB203580 reduced neutrophil migration toward EC junctions. Taken together, these data demonstrate that activation of p38 MAPK, mediated by xanthine oxidase-generated oxidant production, is required for cytoskeletal remodeling in ECs induced by ICAM-1 cross-linking or neutrophil adherence. These cytoskeletal changes in ECs may in turn modulate neutrophil migration toward EC junctions.

Human hypertension caused by mutations in WNK kinases

Hypertension is a major public health problem of largely unknown cause. Here, we identify two genes causing pseudohypoaldosteronism type II, a Mendelian trait featuring hypertension, increased renal salt reabsorption, and impaired K+ and H+ excretion. Both genes encode members of the WNK family of serine-threonine kinases. Disease-causing mutations in WNK1 are large intronic deletions that increase WNK1 expression. The mutations in WNK4 are missense, which cluster in a short, highly conserved segment of the encoded protein. Both proteins localize to the distal nephron, a kidney segment involved in salt, K+, and pH homeostasis. WNK1 is cytoplasmic, whereas WNK4 localizes to tight junctions. The WNK kinases and their associated signaling pathway(s) may offer new targets for the development of antihypertensive drugs.

Regulation of PTEN binding to MAGI-2 by two putative phosphorylation sites at threonine 382 and 383

We have reported previously that the PTEN COOH-terminal 33 amino acids play a role in the maintenance of PTEN protein stability (Tolkacheva and Chan, Oncogene, 19: 680-689, 2000). By site-directed mutagenesis, we identified two threonine residues within this COOH-terminal region at codon 382 and 383 that may be targets for phosphorylation events. Interestingly, PTEN mutants rendered phosphorylation-incompetent at these two sites, T382A/T383A, and were found to have drastically reduced expression in cultured cells. The enhanced degradation of PTEN was most likely mediated by the proteosome-dependent pathway, we have evidence that PTEN was polyubiquitinated. More interestingly, the non-phosphorylated forms of PTEN displayed significantly greater binding affinity than the wild-type protein to a previously identified PTEN interacting partner, MAGI-2/ARIP1. On the basis of all these data, we propose that PTEN recruitment to the cell-cell junction may be regulated through the phosphorylation of its COOH terminus.

Ca(2+) signalling and PKCalpha activate increased endothelial permeability by disassembly of VE-cadherin junctions

1. The role of intracellular Ca(2+) mobilization in the mechanism of increased endothelial permeability was studied. Human umbilical vein endothelial cells (HUVECs) were exposed to thapsigargin or thrombin at concentrations that resulted in similar increases in intracellular Ca(2+) concentration ([Ca(2+)](i)). The rise in [Ca(2+)](i) in both cases was due to release of Ca(2+) from intracellular stores and influx of extracellular Ca(2+). 2. Both agents decreased endothelial cell monolayer electrical resistance (a measure of endothelial cell shape change) and increased transendothelial (125)I-albumin permeability. Thapsigargin induced activation of PKCalpha and discontinuities in VE-cadherin junctions without formation of actin stress fibres. Thrombin also induced PKCalpha activation and similar alterations in VE-cadherin junctions, but in association with actin stress fibre formation. 3. Thapsigargin failed to promote phosphorylation of the 20 kDa myosin light chain (MLC(20)), whereas thrombin induced MLC(20) phosphorylation consistent with formation of actin stress fibres. 4. Calphostin C pretreatment prevented the disruption of VE-cadherin junctions and the decrease in transendothelial electrical resistance caused by both agents. Thus, the increased [Ca(2+)](i) elicited by thapsigargin and thrombin may activate a calphostin C-sensitive PKC pathway that signals VE-cadherin junctional disassembly and increased endothelial permeability. 5. Results suggest a critical role for Ca(2+) signalling and activation of PKCalpha in mediating the disruption of VE-cadherin junctions, and thereby in the mechanism of increased endothelial permeability.

Ras GTPases: singing in tune

A review of the meeting "The Ras Superfamily of Small GTP-Binding Proteins," FASEB Summer Research Conference, Snowmass, Colorado, 15 through 20 July 2000 The molecular cloning of the human proto-oncogene encoding Ras was reported nearly 20 years ago. Since then, Ras has become the prototypical member of a superfamily of small guanosine triphosphatase proteins. Despite the maturity of this field of research, the discovery of new functions and interactions between the superfamily members continues unabated. Symons and Takai have written a meeting report on the latest findings on the Ras superfamily.

Protein kinase Ctheta: a new essential superstar on the T-cell stage

Recent studies have identified protein kinase Cθ (PKCtheta), a member of the Ca(2+)-independent PKC family, as an essential component of the T-cell synapse that cooperates with calcineurin to activate the interleukin-2 (IL-2) gene. Several selective functions of PKCtheta involved in the activation and survival of T cells are reviewed herein. Among these, the nuclear factor-kappaB (NF-kappaB) signaling cascade appears to be the most critical target of PKCtheta in the T-cell receptor/CD28 costimulatory pathway that leads to T-cell activation.

Cell volume-dependent phosphorylation of proteins of the cortical cytoskeleton and cell-cell contact sites. The role of Fyn and FER kinases

Cell volume affects diverse functions including cytoskeletal organization, but the underlying signaling pathways remained undefined. We have shown previously that shrinkage induces Fyn-dependent tyrosine phosphorylation of the cortical actin-binding protein, cortactin. Because FER kinase was implicated in the direct phosphorylation of cortactin, we investigated the osmotic responsiveness of FER and its relationship to Fyn and cortactin. Shrinkage increased FER activity and tyrosine phosphorylation. These effects were abolished by the Src family inhibitor PP2 and strongly mitigated in Fyn-deficient but not in Src-deficient cells. FER overexpression caused cortactin phosphorylation that was further enhanced by hypertonicity. Exchange of tyrosine residues 421, 466, and 482 for phenylalanine prevented cortactin phosphorylation by hypertonicity and strongly decreased it upon FER overexpression, suggesting that FER targets primarily the same osmo-sensitive tyrosines. Because constituents of the cell-cell contacts are substrates of Fyn and FER, we investigated the effect of shrinkage on the adherens junctions. Hypertonicity provoked Fyn-dependent tyrosine phosphorylation in beta-catenin, alpha-catenin, and p120(Cas) and caused the dissociation of beta-catenin from the contacts. This process was delayed in Fyn-deficient or PP2-treated cells. Thus, FER is a volume-sensitive kinase downstream from Fyn, and the Fyn/FER pathway may contribute to the cell size-dependent reorganization of the cytoskeleton and the cell-cell contacts.

Transglutaminase type 1 and its cross-linking activity are concentrated at adherens junctions in simple epithelial cells

Transglutaminase type 1 was identified as a tyrosine-phosphorylated protein from the isolated junctional fraction of the mouse liver. This enzyme was reported to be involved in the covalent cross-linking of proteins in keratinocytes, but its expression and activity in other cell types have not been examined. Northern blotting revealed that transglutaminase type 1 was expressed in large amounts in epithelial tissues (lung, liver, and kidney), which was also confirmed by immunoblotting with antibodies raised against mouse recombinant protein. Immunoblotting of the isolated junctional fraction revealed that transglutaminase type 1 was concentrated in the fraction not only as a 97-kDa form but also as forms of various molecular masses cross-linked to other proteins. In agreement with this finding, endogenous transglutaminase type 1 was immunofluorescently colocalized with E-cadherin in cultured simple epithelial cells. In the liver and kidney, immunoelectron microscopy revealed that transglutaminase type 1 was concentrated, albeit not exclusively, at cadherin-based adherens junctions. Furthermore, by in vitro and in vivo labeling, transglutaminase cross-linking activity was also shown to be concentrated at intercellular junctions of simple epithelial cells. These findings suggested that the formation of covalently cross-linked multimolecular complexes by transglutaminase type 1 is an important mechanism for maintenance of the structural integrity of simple epithelial cells, especially at cadherin-based adherens junctions.

Activation of the protein kinase Akt/PKB by the formation of E-cadherin-mediated cell-cell junctions. Evidence for the association of phosphatidylinositol 3-kinase with the E-cadherin adhesion complex

E-cadherins are surface adhesion molecules localized at the level of adherens junctions, which play a major role in cell adhesiveness by mediating calcium-dependent homophylic interactions at sites of cell-cell contacts. Recently, E-cadherins have been also implicated in a number of biological processes, including cell growth and differentiation, cell recognition, and sorting during developmental morphogenesis, as well as in aggregation-dependent cell survival. As phosphatidylinositol (PI) 3-kinase and Akt play a critical role in survival pathways in response to both growth factors and extracellular stimuli, these observations prompted us to explore whether E-cadherins could affect intracellular molecules regulating the activity of the PI 3-kinase/Akt signaling cascade. Using Madin-Darby canine kidney cells as a model system, we show here that engagement of E-cadherins in homophylic calcium-dependent cell-cell interactions results in a rapid PI 3-kinase-dependent activation of Akt and the subsequent translocation of Akt to the nucleus. Moreover, we demonstrate that the activation of PI 3-kinase in response to cell-cell contact formation involves the phosphorylation of PI 3-kinase in tyrosine residues, and the concomitant recruitment of PI 3-kinase to E-cadherin-containing protein complexes. These findings indicate that E-cadherins can initiate outside-in signal transducing pathways that regulate the activity of PI 3-kinase and Akt, thus providing a novel molecular mechanism whereby the interaction among neighboring cells and their adhesion status may ultimately control the fate of epithelial cells.

The fate of E- and P-cadherin during the early stages of apoptosis

Caspases are responsible for the proteolysis of many cytoskeletal proteins in apoptotic cells. It has been demonstrated here that during cisplatin-induced apoptosis of human embryo retinoblasts both E- and P-cadherin were degraded by caspases, giving initially major polypeptide products of apparent molecular weights 48 K and 104 K respectively. This proteolysis occurred over a similar time-scale to the observed degradation of PARP and to the onset of DNA fragmentation but appreciably later than p53 induction and cleavage of Mdm2 and p21. Addition of caspase inhibitors such as Z-VAD-FMK inhibited apoptosis and cadherin degradation. Co-immunoprecipitation studies carried out on viable cells confirmed previously observed complexes between cadherins and alpha and beta catenin and between the catenins themselves. These interactions were sustained in apoptotic cells as long as the protein components remained intact. Using confocal microscopy it has been shown that cytoskeletal changes associated with apoptosis precede degradation of catenins and cadherins by several hours. In particular, after addition of cisplatin relatively rapid (within 3 h) re-localization of adherens junction proteins from the cell periphery to the cytoplasm was observed whereas little cadherin or catenin degradation occurred until 10 h. We conclude that neither caspase-mediated degradation of cytoskeletal components nor disruption of adherens junction protein-protein interactions is required for morphological change.

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.

"Cytosolic" phospholipase A2 is in the nucleus of subconfluent endothelial cells but confined to the cytoplasm of confluent endothelial cells and redistributes to the nuclear envelope and cell junctions upon histamine stimulation

The synthesis of arachidonic acid metabolites is initiated by activation of the sn-arachidonyl-dependent, 85-kd "cytosolic" phospholipase A2 (cPLA2) enzyme. We have investigated the subcellular localization of cPLA2 in resting and histamine-treated human and bovine endothelial cells (EC) using confocal immunofluorescence microscopy. In tightly confluent EC, cPLA2 was primarily localized in the cytoplasm. Surprisingly, in subconfluent EC, cPLA2 was also prominently located within the cell nucleus. By immunoblotting of cell lysates after SDS-PAGE, the cytoplasmic molecular species in subconfluent cells displayed the characteristic Mr 110,000, whereas nuclear extracts contained a predominant Mr 70,000. Nuclear localization of cPLA, in subconfluent EC is independent of cell cycle because it was observed in growth-arrested cells as well as in dividing cells. Nuclear localization was also observed in subconfluent MDCK and HeLa cells where, in contrast to EC, it persisted in tightly confluent monolayers. Treatment of subconfluent EC with histamine caused a rapid, dose-dependent redistribution of cPLA2, from the nucleus to the nuclear envelope. The same treatment of confluent EC produced translocation of cytoplasmic enzyme to the nuclear envelope and to the plasma membrane at the intercellular junctions. The cell density dependence of cPLA2, localization may contribute to altered arachidonic acid metabolism in injured vessels as compared with quiescent vessels.

Immunolocalization of Na,K-ATPase in blowfly photoreceptor cells

The Na,K-ATPase (sodium pump) plays a central role in the physiology of arthropod photoreceptors as it re-establishes gradients for Na+ and K+ after light stimulation. We have mapped the distribution of the Na,K-ATPase in the photoreceptors of the blowfly (Calliphora erythrocephala) by immunofluorescent and immunogold cytochemistry, and demonstrate that the distribution pattern is more complex than previously presumed. High levels of sodium pumps have been detected consistently in all photoreceptors R1-8 on the nonreceptive surface, but no sodium pumps are found on the microvillar rhabdomere. Within the nonreceptive surface of the cells R1-6, however, the sodium pumps are confined to sites juxtaposed to neighboring photoreceptor or glial cells; no sodium pumps have been detected on the parts of the nonreceptive surface exposed to the intra-ommatidial space. In R7 and R8, the sodium pumps are found over the entire nonreceptive surface. The cytoskeletal protein spectrin colocalizes with the sodium pumps suggesting that linkage of the pump molecules to the spectrin-based submembrane cytoskeleton contributes to the maintenance of the complex pattern of pump distribution.

A novel phosphoglucomutase-related protein is concentrated in adherens junctions of muscle and nonmuscle cells

Using five monoclonal antibodies raised against a human uterine smooth muscle extract, we have identified a novel antigen which runs as a closely spaced doublet in SDS-gels. The proteins (60/63 kDa) co-purify, are present in a 1:1 ratio as judged by Coomassie Blue staining, and are immunologically closely related, if not identical. No N-terminal sequence could be obtained from a mixture of the 60/63 kDa proteins, but the sequence of four polypeptides liberated by V8 protease or cyanogen bromide cleavage showed that the proteins are closely related to the glycolytic enzyme phosphoglucomutase type 1. Affinity-purified polyclonal antibodies and three different monoclonal antibodies to the 60/63 kDa proteins cross-reacted with rabbit skeletal muscle phosphoglucomutase type 1, whilst two additional monoclonal antibodies were specific for the 60/63 kDa proteins. Peptide maps of the 60/63 kDa proteins and phosphoglucomutase 1 are markedly different, and the purified proteins have no detectable phosphoglucomutase activity. Staining of cultured smooth muscle cells and fibroblasts with antibodies to 60/63 kDa proteins showed that the antigen is concentrated in focal contacts at the ends of actin bundles and is also associated with actin filaments. About 60% of the cellular 60/63 kDa proteins were found in the detergent-insoluble fraction, suggesting a physical association with the cytoskeleton. The highest levels of protein immunoreactivity were found in muscles. The antigen is concentrated in muscle adherens junctions, including smooth muscle dense plaques, cardiomyocyte intercalated disks, and striated muscle myotendinous junctions. Among epithelial cells, the 63 kDa isoform of the protein was found only in cultured keratinocytes where immunofluorescent staining was localized in cell-to-cell adherens junctions. Expression of the 60/63 kDa proteins in vascular smooth muscle cells is developmentally regulated and correlates with the differentiated contractile phenotype of these cells.

Morphological aspects of transport of potassium ion in the marginal cell

Structure and function of the marginal cell in the stria vascularis were studied by freeze-fracture, cytochemistry and immunohistochemistry with special regard to the ion transport of potassium. Freeze-fracture showed that marginal cells were connected by tight junctions beneath the scala media K(+)-NPPase cytochemistry showed that Na+, K(+)-ATPase was abundant on the basolateral infoldings of the marginal cell. Immunohistochemistry of a rat Isk protein, which has a property of a potassium channel, revealed that the rat Isk protein was localized at the endolymphatic surface of the marginal cell. These findings supported the 'one-pump' theory (Offner et al. Hear Res 1987; 29: 117-24).

Acid phosphatase localization in endocytosed horizontal cell gap junctions

Gap junction (GJ) endocytosis appears to be part of a cycle of GJ renewal in horizontal cells of the teleost fish retina. At least three stages of GJ endocytosis in these neurons have been identified using conventional electron microscopy (EM): invagination of GJ membranes (GJ blebs); free GJ vesicles; and GJ vesicle fusion with mature lysosomes (Vaughan & Lasater, 1990a). In the present study, EM-level acid phosphatase (AP) histochemistry of white bass retina was used to determine at what stage enzymatic degradation of endocytosed GJs begins. Electron-dense AP reaction product was observed within the trans face of the Golgi apparatus, mature lysosomes, and occasional, internal GJ vesicles. In contrast, GJ blebs, peripheral GJ vesicles, and most internal GJ vesicles lacked AP reaction product. These results support the idea that at least some of the GJ vesicles observed within these retinal neurons arise from endocytosis, are on a degradative pathway, and can be termed GJ "endosomes." Furthermore, GJ vesicles appear to be initially free of AP, but some later acquire it (presumably from transport vesicles bearing degradative enzymes). It is still unclear whether our previous report of GJ vesicle fusion with mature lysosomes is a subsequent step in GJ degradation or part of a different degradative pathway altogether.

Ultrastructural localization of acetylcholinesterase in the guinea pig pineal gland

The ultrastructural localization of acetylcholinesterase (AChE) activity in guinea pig pineal gland was studied using the copper-glycine procedure. A small number of pinealocytes and bundles of unmyelinated nerve fibers were labeled by the AChE reaction. The AChE-positive pinealocytes were located near blood vessels and distributed in small groups. The AChE reaction product was localized in the perinuclear cistern, in the cisternae of the endoplasmic reticulum (ER), and in the saccules of the Golgi apparatus. These findings suggest that the AChE-positive pinealocytes synthesize AChE. The AChE reaction product was also seen in the intercellular space between pinealocyte processes. Besides pinealocytes, AChE activity was localized on the axolemma of myelinated and unmyelinated nerve fibers and in the basement membrane surrounding unmyelinated nerve fibers. Pseudocholinesterase activity was confined to Schwann cells, which showed the reaction product in their perinuclear cistern, in the cisternae of the ER, and on the plasmalemma.

Effect of isoproterenol and 3-isobutyl-1-methylxanthine on junctional conductance in heart cell pairs

The influence of isoproterenol (10(-6) M) and 3-isobutyl-1-methylxanthine (IBMX) (10(-6) M) on the junctional conductance (gj) of isolated rat ventricular cell pairs was investigated. It was found that both drugs increased gj within 25-30 s. To test the phosphorylation hypothesis, cAMP-dependent protein kinase inhibitor (20 micrograms/ml) was dialyzed into both cells of the pair. The protein kinase inhibitor suppressed the effect of both drugs on gj. Moreover, the protein kinase inhibitor by itself, reduced gj by 18% (S.E. +/- 9) (n = 10), suggesting that basal levels of cAMP in the cells contributes to modulation of gj. Dialysis of cAMP-protein kinase inhibitor (20 micrograms/ml) in just one cell of the pair induced rectification of the junctional membrane. These results indicate that the activation of cAMP-dependent protein kinase is necessary for the effect of cAMP on junctional conductance in heart.

A 16 kDa protein co-isolating with gap junctions from brain tissue belonging to the class of proteolipids of the vacuolar H+-ATPases

A 16 kDa protein from an enriched gap junction preparation was isolated from bovine brain tissues. N-terminal amino acid microsequencing of the first 20 amino acids showed a complete homology with a recently published sequence of a proteolipid from a vacuolar H+-ATPase from chromaffin granules. Incubation of the brain gap junction preparation with 14C-N,N'-dicyclohexylcarbodiimide showed a significant binding of this compound to the 16 kDa protein, indicating that a proton binding site also occurs within that particular protein. The data suggest that this 16 kDa protein, which has also been described in gap junction preparations from various other tissues, belongs to the proton transporting ATPase.

Effects of microgravity on liposome-reconstituted cardiac gap junction channeling activity

Effects of microgravity on cardiac gap junction channeling activity were investigated aboard NASA zero-gravity aircraft. Liposome-reconstituted gap junctions were assayed for channel function during free-fall, and the data were compared with channeling at 1 g. Control experiments tested for 0 g effects on the structural stability of liposomes, and on the enzyme-substrate signalling system of the assay. The results demonstrate that short periods of microgravity do not perturb reconstituted cardiac gap junction channeling activity.

Fine structure of myomyous junctions in the mouse skeletal muscles

The reaction product of acetylcholinesterase (AChE) activity is known to be specifically localized at a neuromuscular junction and a muscle-tendon junction of the striated skeletal muscles. In addition to the two junctions, we recently found some linear precipitates due to AChE activity running transversely across a fibre of the semitendinosus, rectus abdominis, gastrocnemius, tibialis anterior and diaphragm muscles in mice. Under an electron microscope, the linear precipitates were seen at the extracellular side of the muscle fibre endings. Most of the endings contacted each other to form a junction, which has been called the 'myomyous junction (M-Mj)'. The patterns of the M-Mj were grouped into three types: (1) a junction in which all contacts were firm, without any connective tissue, and invaginated deeply; (2) the ones in which numerous collagen fibres were visible in the space between the two separate opposing muscle fibres; (3) an intermediate type between (1) and (2), i.e. a junction with partial contacts. The muscle fibre ending forming M-Mj was constructed of finger-like processes like that of a muscle-tendon junction. However, the processes of a M-Mj adhered so closely to each other that no collagen fibrils could penetrate into their folds.

Calmodulin-dependent protein phosphatase: immunocytochemical localization in chick retina

Calmodulin-dependent protein phosphatase, previously called CaM-BP80 or calcineurin, is present in high concentrations in the central nervous system. The level of the phosphatase has been shown by radioimmunoassay to increase during development in the retinas of embryonic and hatching chicks (Tallant, E.A., and W.Y. Cheung, 1983, Biochemistry, 22:3630-3635). The aims of this study are to immunocytochemically localize the phosphatase in developing and mature retinas and to determine if the phosphatase is present in fractions of retinal synaptic membranes and synaptic junctions. Vibratome slices of fixed chick retina and Western blots of detergent-solubilized retinal fractions are both treated sequentially with rabbit primary antisera and goat anti-rabbit Fab fragments conjugated to peroxidase, and then reacted with hydrogen peroxide and diaminobenzidine. The tissue slices are further processed for electron microscopy. This paper demonstrates the presence of peroxidase reaction product in the retina just before synapse formation. In the outer plexiform layer the product is confined to photoreceptor synaptic terminals, whereas in the inner plexiform layer it is present in synaptic terminals of bipolar cells and in dendrites of ganglion cells. In this latter site the product is present postsynaptically at bipolar and amacrine synapses. The phosphatase is detected in Western blots of both synaptic plasma membrane and synaptic junction fractions.

The axosomatic contacts on the bursting neuron of the snail Helix pomatia. II. Ultrastructural localization of adenylate cyclase

Fluoride and peptide-stimulated adenylate cyclase activity was investigated by electron histochemistry on serial sections of the RPAI neuron of the snail Helix pomatia. Fluoride-stimulated adenylate cyclase was detected in the surface membrane of the RPAI neuron, the postsynaptic membrane of axosomatic contacts, and the surface of glial cells forming a multilayer capsule around the neuron. Peptide-stimulated adenylate cyclase was located in the membrane of glial cells surrounding the neuron, their processes (trophospongia) invaginating deeply in the neuronal soma, and the membrane of somatic protrusions forming the system of lacoons in the region of the axosomatic contact. No peptide-stimulated adenylate cyclase was revealed in the remaining part of the surface of the somatic membrane. The localization of adenylate cyclase activity in the postsynaptic membrane in the region of the axosomatic contact is in accordance with the hypothesis based on electrophysiological experiments that the cyclase system participates in the genesis and regulation of the bursting activity of the RPAI neuron.

Ultracytochemical localization of Ca++-ATPase activity in the paraphyseal epithelial cells of the frog, Rana esculenta

Ca++-ATPase activity was studied ultracytochemically (cf. Ando et al. 1981) in the paraphysis cerebri of the frog. An intense reaction was demonstrated on the plasmalemma of the microvilli at the apical pole of paraphyseal cells; in contrast, the basolateral plasmalemma showed only a slight staining. In addition, mitochondria, gap junctions, cilia, and cytoplasmic elements (e.g., microfilaments) displayed Ca++-ATPase activity. Variation of the Ca++-concentration in the incubation medium from 0.1 mM to 100 mM altered the Ca++-ATPase activity of the cell organelles. The substitution of Ca- by Mg-ions resulted in a conspicuous decrease in the enzyme activity, especially on the apical plasmalemma. Ca++-ATPase activity is claimed to be involved in a number of extra- and intracellular functions. In comparison to the epithelium of the adjacent choroid plexus the paraphyseal epithelial cell is thought to be a principal Ca-ion regulator of the cerebrospinal fluid in frogs.

Localization of acetylcholinesterase in the rabbit cornea by light and electron microscopy

Acetylcholinesterase (AChE) has been localized in the rabbit cornea by light and electron microscopy histochemical techniques. In the stroma, the enzyme is concentrated in nerves. In the epithelium, the enzyme is concentrated in intercellular spaces devoid of nerves. The morphologic appearance of the enzyme staining by light and electron microscopy in the epithelium is similar; consequently, the staining demonstrated with light microscopy examination does not always represent epithelial nerves. A significant portion of corneal acetylcholinesterase therefore appears unrelated to nerves. Considerably smaller deposits of enzyme reaction product were present in cells in every layer of the cornea, using electron microscopy histochemistry; they were not identified by light microscopy.

Surface ATPase activity at cell-cell contacts in hepatic parenchymal cells and in cAMP-treated hepatoma cells in monolayer culture

Histochemical investigation shows that ATPase activity is located intensively on the surface of cell contacts in hepatoma cells grown in confluent monolayer culture. Dibutyryl cyclic AMP and theophylline-treated hepatoma cells which exhibit contact-inhibited growth show the absence of localization of intense ATPase activity at cell-cell contacts. However, after removal of these additives the activity fully recovers to the intense level of control cells, when the release of cells from contact inhibition of growth occurs. Cultured hepatic parenchymal cells in monolayer have little or no ATPase activity at their surface immediately after contacts are established, and again after growth to a confluent state. In a different type of hepatoma cell which is less malignant and forms a small tissue mass or island, cell surface ATPase activity at cell-cell contacts in an island is very weak or scarcely detected even when cells are not treated with dibutyryl cyclic AMP and theophylline.

Isolation of synaptic junctional complexes of high structural integrity from rat brain

A new method has been developed for isolating synaptic junctional complexes (SJC) of high structural integrity. The major step in the isolation involves homogenization of a synaptosomal membrane (SM) fraction in a biphasic system consisting of Freon 113 and an aqueous phase containing 0.2% Triton X-100. Well-preserved SJCs, along with membrane vesicles, were recovered in the aqueous phase after low-speed centrifugation of the homogenate. The membranes were subsequently separated from the SJCs by centrifugation on a discontinuous sucrose density gradient. The purity and identity of subcellular fractions were monitored by thin sectioning electron microscopy, using specific and nonspecific staining methods. From the electron microscope studies we conclude that SJCs and their components occupy about 65% of the area covered by structures in this fraction. The assay of enzyme activities indicates that homogenization in Triton-Freon and subsequent steps of the isolation procedure affect the activities of Na, K-ATPase, cytochrome oxidase, and acid phosphatase to different extents, but do not cause total inactivation. Electrophoresis of the SJC-enriched fraction on sodium dodecyl sulfate-polyacrylamide gels has demonstrated that a polypeptide which co-migrates with tubulin is the major component in this fraction, and that a polypeptide co-migrating with actin is also present.

Formation of myoneural and myotendinous junctions in the chick embryo. Role of acetylcholinesterase-rich granules in the developing muscle fibers

The mode of formation of the myoneural and myotendinous junctions was investigated in the thigh muscles of the chick embryo. Myotendinous junctions first appeared on day 11 of incubation, whereas myoneural junctions developed on day 12. Intracellular AChE activity in the muscles increased by the 12th day of incubation, and decreased rapidly after the formation of the myoneural junctions. Light and electron microscopically, AChE activity was demonstrated in the nuclear envelope, sarcoplasmic reticulum, Golgi complex, and in large granules which appeared to be derived from the Golgi complex. Large granules showing an intense AChE activity accumulated in the sarcoplasm at the poles of the muscle fiber before the formation of myotendinous junctions. After the translocation of this intracellular enzyme onto the sarcolemma, most likely the result of an exocytosis of the granules, the myotendinous junctions were formed. The AChE-rich granules present in the middle of myotubes developed into spindle- or comma-shaped cisternae which were located in the sarcoplasm just below the presumptive motor endplates. The present results suggest that the transport of AChE-rich granules to the sarcolamma is the first step in the formation of myoneural and myotendinous junctions.