Cell junction and cyclic AMP: I. Upregulation of junctional membrane permeability and junctional membrane particles by administration of cyclic nucleotide or phosphodiesterase inhibitor

Summary Mammalian cells in culture were exposed to cyclic AMP, dibutyrul cyclic AMP, the phosphodiesterase inhibitor caffeine, or a combination of the last two, while junctional molecular transfer was probed with the series of microinjected, fluorescentlabelled linear molecules Glu, Glu-Glu, Glu-Glu-Glu, and Leu-Leu-Leu-Glu-Glu. The junctional permeability for these molecules increased with each of the agents, most markedly with the dibutyryl cyclic AMP-caffeine combination, as the intracellular cyclic nucleotide concentration rose. The junctional permeability effect developed over several hours. When probed with molecules close to the limit of cell-to-cell channel permeation (the most sensitive setting), the effect was detectable both, as an increase in the (relative) junctional transit rate and as an increase in the number of transferring cell interfaces in the test populations. The number of transferring cell interfaces reached a maximum by 4 hr, when the junctional transit rate, hence the junctional permeability, was still rising. Nonjunctional membrane permeability for the probe molecules, as determined by intracellular fluorescence loss, was not significantly changed (nor was there significant nonjunctional cell-to-cell transfer of molecules before or after the treatments). The rise in junctional permeability was associated with an increase in the number of gap junctional membrane particles, as determined by freeze-fracture electron microscopy: the average size of the particle clusters increased, and the frequency of the clusters increased, particularly that of the smaller (and presumably newer) clusters. This effect was blocked by treatments with the protein synthesis inhibitors cycloheximide or puromycin. These agents caused particle diminution (diminution of cluster frequency but not of average cluster size), with or without cyclic nucleotide. The junctional effects may represent a cyclic AMP-promoted proliferation of cell-to-cell channels. Some physiological implications, in particular, implications for hormone-regulated tissues, are discussed.     

Hormonal regulation of cell junction permeability: Upregulation by catecholamine and prostaglandin E1

Summary By cellular activation with hormones, we test the proposition (Loewenstein, W.R.,Physiol. Rev. 61:829, 1981) that the permeability of cell junction is upregulated through elevation of the level of cyclic AMP. Cultured rat glioma C-6 cells, with -adrenergic receptors, and human lung WI-38 cells, with prostaglandin receptors, were exposed to catecholamine (isoproterenol) and prostaglandin E₁, respectively, while their junctions were probed with microinjected fluorescent-labelled mono-, di-, and triglutamate. Junctional permeability, as indexed by the proportion of cell interfaces transferring the probes, rose after the hormone treatments. The increase in permeability took several hours to develop and was associated with an increase in the number of gap-junctional membrane particles (freeze-fracture electron microscopy). Such interaction between hormonal and junctional intercellular communication may provide a mechanism for physiological regulation of junctional communication and (perhaps as part of that) for physiological coordination of responses of cells in organs and tissues to hormones.     

The permeability of the cell-to-cell membrane channel and its regulation in mammalian cell junctions

Mammalian cell-to-cell channels show polar permselective properties discriminating against negatively charged 14 A-wide molecules and are more restrictive than the channels of insect cell junctions. The channel permeability is modulated by conditions affecting the concentration of intracellular ionic Ca: elevation of the external Ca load (B cells), treatment of cell cultures with Ca-transporting ionophore (in the presence of external Ca, but not in its absence), treatment with a combination of cyanide and iodoacetate, or with high levels of carbon dioxide, all cause depression of channel permeability. Treatment of cell cultures with cyclic AMP or its more permeable derivative, dibutyryl cyclic AMP, produces increase in permeability. A similar channel up regulation is observed upon elevation of the endogenous level of cyclic AMP by serum deprivation or lowering of cell density.     

Permeability of the cell-to-cell membrane channel and its regulation in an insect cell junction

Summary Cells of organs and tissues commonly communicate directly with one another via permeable membrane junctions. Cell-to-cell channels, spanning the width of both membranes of a junction, are thought to provide the pathways between the cytoplasms of adjacent cells for the immediate exchange of ions and small molecules. We study these cell-to-cell channels in a cell model system, the salivary gland ofChironomus. Using intracellularly injected fluorescent labelled peptides and oligosaccharides of various molecular dimensions as channel permeability probes we find the channels to have a bore of about 2 nm. The channel permeability can be modulated and, in the extreme, the channels can be closed under various experimental conditions. With the aid of the Ca²⁺-sensitive photoprotein aequorin as monitor of cytoplasmic free Ca²⁺ concentration, we show that a determining factor in this modulation of channel permeability is the cytoplasmic free Ca²⁺ concentration. Moreover, results obtained by injection of different-sized and different-labelled channel permeability probes together with Ca²⁺ indicate that closure of the individual channels may occur in more than one step, i.e., by a graded reduction of channel bore. Presented in the symposium on Molecular and Morphological Aspects of Cell-Cell Communication at the 31st Annual Meeting of the Tissue Culture Association, St. Louis, Missouri, June 1–5, 1980. This symposium was supported, in part by Contract 263-MD-025754 from the National Cancer Institute and the Fogarty International Center. This work was supported by NH Grants 5P1GM23911-07 and 5T32-6M07403-04.     

Permeability properties of cell-to-cell channels: Kinetics of fluorescent tracer diffusion through a cell junction

Summary We have analyzed the intracellular and cell-to-cell diffusion kinetics of fluorescent tracers in theChironomus salivary gland. We use this analysis to investigate whether membrane potential-induced changes in junctional permeability are accompanied by changes in cell-to-cell channel selectivity. Tracers of different size and fluorescence wavelength were coinjected into a cell, and the fluorescence was monitored in this cell and an adjacent one. Rate constants,k_j, for cell-to-cell diffusion were derived by compartment model analysis, taking into account (i) cell-to-cell diffusion of the tracers; (ii) their loss from the cells; (iii) their binding (sequestration) to cytoplasmic components; and (iv) their relative mobility to cytoplasm, as determined separately on isolated cells. In cell pairs, we compared a tracer'sk_j with the electrical cell-to-cell conductance,g_j.At cell membrane resting potential, thek_j's ranged 3.8–9.2×10^–3 sec^–1 for the small carboxyfluorescein (mol wt 376) to about 0.4×10^–3 sec^–1 for a large fluorescein-labeled sugar (mol wt 2327). Cell membrane depolarization reversibly reducedg_j andk_j for a large and a small tracer, all in the same proportion. This suggests that membrane potential controls the number of open channels, rather than their effective pore diameter or selectivity. From the inverse relation between tracer mean diameter and relativek_j we calculate an effective, permeation-limiting diameter of approximately 29 Å for the insect cell-to-cell channel.Intracellular diffusion was faster than cell-to-cell diffusion, and it was not solely dependent on tracer size. Rate constants for intracellular sequestration and loss through nonjunctional membrane were large enough to become rate-limiting for cell-to-cell tracer diffusion at low junctional permeabilities.     

Structure and permeability of goblet cell tight junctions in rat small intestine

Summary Two major cell types, goblet and absorptive cells, dominate the epithelial lining of small intestinal villi. We used freezefracture replicas of rat ileal mucosa to examine the possibility that tight junction structure, known to relate to transepithelial resistance, might vary with cell type. Tight junctions between absorptive cells were uniform in structure while those associated with villus goblet cells displayed structural variability. In 23% of villus goblet cell tight junctions the strand count was less than 4 and in 30% the depth was less than 200 nm. In contrast, only 4% of absorptive cell tight junctions had less than 4 strands and only 9% had depth measurements less than 200 nm. Other structural features commonly associated with villus goblet cell tight junctions but less commonly with absorptive cell tight junctions were: deficient strand cross-linking, free-ending abluminal strands, and highly fragmented strands. Bothin vivo ileal segments and everted loops were exposed to ionic lanthanum. Dense lanthanum precipitates in tight junctions and paracellular spaces were restricted to a subpopulation of villus goblet cells and were not found between villus absorptive cells. After exposure of prefixed ileal loops to lanthanum for 1 hour, faint precipitates of lanthanum were found in 14% of tight junctions and paracellular spaces between absorptive cells compared to 42% of tight junctions and paracellular spaces adjacent to villus goblet cells. When tested in Ussing chambers, the methods used for lanthanum exposure did not lower transepithelial resistance. Everted loops exposed to ionic barium and examined by light microscopy showed dense barium precipitates in the junctional zone and region of the paracellular space of villus goblet cells but not in these regions between absorptive cells. However, the macromolecular tracers, microperoxidase, cytochromec and horseradish peroxidase, were excluded from both villus goblet cell and absorptive cell paracellular spaces inin vivo segments. These findings suggest that a subpopulation of villus goblet cells may serve as focal sites of high ionic permeability and contribute to the relatively low resistance to ionic flow which characterizes the small intestinal epithelium.     

Intercellular communication and the control of growth: XII. Alteration of junctional permeability by simian virus 40. roles of the large and smallT antigens

Summary We studied the action of temperature-sensitive mutant simian virus 40—a transformation-inducing DNA virus—on the junctional permeability to mono-, di- and triglutamate in rat embryo-, pancreas islet (epithelia)-, and 10T1/2 cell cultures. Junctional permeability was reduced (reversibly) in the transformed state. To dissect the genetics of this alteration, we used two kinds of mutant virus DNA. One kind had a temperature-sensitive mutation on theA gene, rendering the largeT antigen (the gene product) thermolabile (T ⁺ T ^–). The other had a deletion on theF gene, in addition, abolishing (permanently) the expression of the littlet antigen (t ^–). The junctional alteration occurred in the conditionT ⁺ t ⁺, but not in the conditionsT ^– t ⁺,T ⁺ t ^– orT ^– t ^–. Both antigens, thus, are necessary for this junctional alteration—a genetic requirement identical to that for decontrol of growth (but distinct from that of the cytoskeletal alteration).     

Multiple effects of mercury on cell volume regulation, plasma membrane permeability, and thiol content in the human intestinal cell line Caco-2

In a previous study, we characterized Cd–Hg interactions for uptake in human intestinal Caco-2 cells. We pursued our investigations on metal uptake from metal mixtures, focusing on the effects of Hg on cellular homeostasis. A 4-fold higher equilibrium accumulation value of 0.3 μmol/L ²⁰³Hg was measured in the presence of 100 μmol/L unlabeled Hg in the serum-free exposure medium without modification in the initial uptake rate. This phenomenon was eliminated at 4^∘C. Mercury induced an increase in tritiated water and [³H]mannitol uptakes for exposure times greater than 20 min. Incubations for 20 min and 30 min with 100 μmol/L Hg and 2 mmol/L N-ethylmaleimide (NEM) resulted in a 34% and 50% reductions in cellular thiol staining, respectively, with additive effects. Lactate dehydrogenase leakage and live/dead assays confirmed the maintenance of cell membrane integrity in Hg- or NEM-treated cells. We conclude that Hg may alter membrane permeability and increase cell volume without any loss in cell viability. This phenomenon is sensitive to temperature and could involve Hg interaction with membrane thiols, possibly related to solute transport. During metal uptake from metal mixtures, Hg may thus promote the uptake of other toxic metals by increasing cell volume and consequently cell capacity. Deceased 25 March 2004     

Intercellular communication and the control of growth: XI. Alteration of junctional permeability by thesrc gene in a revertant cell with normal cytoskeleton

Summary To learn whether the reduction of cell-to-cell communication in transformation is a possible primary effect of pp60^src phosphorylation or secondary to a cytoskeletal alteration, we examined the junctional permeability in transformed cells with normal cytoskeleton. The permeability to fluorescentlabelled mono- and diglutamate was compared in clones of Faras' vole cells—clones transformed by Rous sarcoma virus and reverted from that transformation. One revertant clone (partial revertant), had the high levels of pp60^src kinase activity and tumorigenicity of the fully transformed parent clone, but had lost the cytoskeletal alterations of that clone. Another revertant clone (full revertant) had lost the tumorigenicity and most of the pp60^src kinase activity, in addition (J.F. Nawrocki et al., 1984,Mol. Cell Biol.4:212). The junctional permeability of thepartial revertant with normal cytoskeleton was similar to that of the fully transformed parent clone with abnormal cytoskeleton. The permeabilities of both were lower than those of thefull revertant and the normal uninfected cell, demonstrating that the junctional change by thesrc gene is independent of the cytoskeletal one.     

Chloride permeability in human red cells: Influence of membrane protein rearrangement resulting from ATP depletion and calcium accumulation

Summary As 15% of band 3 protein, the assumed chloride channel, is associated with spectrin, the major peripheral protein of a lattice located at the red cell membrane-cytosol interface, the present study was undertaken to evaluate whether a rearrangement of the lattice modifies the functional property of band 3 protein. Such a rearrangement was modulated by depletion of cell ATP and/or by accumulation of Ca²⁺ ions within the cell.ATP depletion induces an inhibition of the electroneutral one-for-one chloride exchanges. Neither the modification of red cell morphology due to ATP depletion (discocyte-echinocyte transformation) nor a direct effect of the decrease in internal ATP level can account for this inhibition. On the other hand, it seems reasonable to consider that inhibition is related to the changes in membrane protein organization (formation of heteropolymers) induced by the decrease in ATP level. But it does not appear that the degree of inhibition is modified when this altered assembly of membrane protein is stabilized by disulfide linkages.Accumulation of Ca²⁺ ions in the cell at a relatively low concentration (10m range) inhibits chloride exchange without apparent modification of the assembly of membrane proteins. This effect of calcium on chloride exchanges is speculatively denoted as a direct effect of calcium.Calcium loading of fresh red cells at higher concentrations (500 to 1000 m) obtained by use of the ionophore A23187 induces a very strong inhibition of chloride exchanges. In this case, inhibition can be reasonably accounted for by two simultaneous effects of calcium: a direct effect which explains half of the inhibition and an indirect effect due to the formation of membrane protein complexes stabilized by covalent crosslinkages (activation by Ca²⁺ ions of a transglutaminase).It is interesting to note that intracellular calcium, whatever the level, inhibits electroneutral exchanges of chloride but increases net chloride movements.     

The possible influence of osmotic poration on cell membrane water permeability

It has been hypothesized that pores in the plasma membrane form under conditions of rapid water efflux, allowing extracellular ice to grow into the cytoplasm under conditions of rapid freezing. When cells with intracellular ice are thawed slowly, the transmembrane ice crystal expands through recrystallization causing the cell to lyse. One of the implications of this hypothesis is that osmotic pores will provide an alternative route for water movement under conditions of osmotically induced flow. We show that the plasma membrane water permeability of a fibroblast cell changes as a function of the osmotic pressure gradient that is used to drive water movement. It is further shown that cell volume is more important than the magnitude of water flux in causing this departure from a uniform water permeability. We suggest that these data provide evidence of a transient route for water movement across cell membranes.     

The permeability of the epidermal cell plasma membrane of barley leaves to abscisic acid

Uptake of ³H-labelled (±)-abscisic acid (ABA) into isolated barley (Hordeum vulgare L.) epidermal cell protoplasts (ECP) was followed over a range of pH values and ABA concentrations. The present results show that ABA uptake is not always linearly correlated with the external concentration of undissociated ABA (ABAH). At pH 7.25, ABA uptake exhibited saturation kinetics with an apparent K _m value of 75 mmol·m^–3 to tal ABA. This saturable transport component was inhibited by pretreating the protoplasts with 1 mol·m^–3 p-chloromercuribenzenesulfonic acid at pH 8.0, conditions that minimized the uptake of this acid sulfhydryl reagent. Moreover, the rate of (±)-[^3]HABA uptake was reduced by addition of 0.1 mol·m^–3 (±)-ABA to 41%, whereas the same concentration of (±)-ABA was approximately half as effective (46% of the inhibitory effect). Thus, it was concluded that only (±)-ABA competes for an ABA carrier that is located in the epidermal cell plasma membrane. The permeability of the epidermal cell plasma membrane was studied by performing a Collander analysis. At pH 6 the overall plasma-membrane permeability of epidermal cells was similar to that of guard cells but was about two times higher than that of mesophyll cells.Abbreviations ABA abscisic acid - ABA^– anion of ABA - ABAH undissociated ABA - 2,4-D 2,4-dichlorophenoxyacetic acid - DMO 5,5-dimethyloxazolidine-2,4-dione - ECP deepidermal cell protoplast - K_r partition coefficient - M_r relative molecular mass - NEM N-ethylmaleimide - PCMBS p-chloromercuriben zenesulfonic acid - P_s permeability coefficient We are grateful to Barbara Dierich for expert technical assistance, to Prof. H. Gimmler (Lehrstuhl für Botanik I, Universität Würzburg, FRG) for helpful discussions and to the Deutsche Forschungsgemeinschaft (SFB 251, TP 3) for financial support.     

Regulation of the mitochondrial outer membrane channel,VDAC

The channel-forming protein, VDAC, located in the mitochondrial outer membrane, is probably responsible for the high permeability of the outer membrane to small molecules. The ability to regulate this channelin vitro raises the possibility that VDAC may perform a regulatory rolein vivo. VDAC exists in multiple, quasi-degenerate conformations with different permeability properties. Therefore a modest input of energy can change VDAC's conformation. The ability to use a membrane potential to convert VDAC from a high (open) to a low (closed) conducting form indicates the presence of a sensor in the protein that allows it to respond to the electric field. Titration and modification experiments point to a polyvalent, positively charged sensor. Soluble, polyvalent anions such as dextran sulfate and Konig's polyanion seem to be able to interact with the sensor to induce channel closure. Thus there are multiple ways of applying a force on the sensor so as to induce a conformational change in VDAC. Perhaps cells use one or more of these methods.     

谷氨酰胺对心肌细胞线粒体膜PTP开放干预作用的实验研究

目的:探讨谷氨酰胺(Gln)对过度训练状态下心肌线粒体膜通透性转换孔(PTP)开放的干预作用及其可能机制。方法:30只SD大鼠随机分为3组(n=10):对照组(CG组)、过度训练组(OG组)和补充Gln+过度训练组(GOG组)。采用分光光度法检测大鼠心肌线粒体PTP开放程度,电化学法检测心肌丙二醛(MDA)、还原型谷胱苷肽(GSH)含量和磷脂酶A2(PLA2)活性。结果:OG组与GOG组比较,吸光度(A0)显著下降(P<0.05),吸光度变化(△A)值显著降低(P<0.05);荧光剂罗丹明123(Rh123)的荧光强度(F0)显著增强(P<0.05),Rh123荧光强度变化(△F)值明显降低(P<0.05)。与GOG组比较,线粒体GSH含量显著降低(P<0.05),PLA2活性显著增加(P<0.05);MDA含量显著升高(P<0.05)。结论:过度训练可导致心肌细胞线粒体PTP开放增加,过度训练状态下线粒体活性氧生成增多,PLA2活性增加及GSH的含量下降,补充外源性的Gln对这些变化有显著的干预作用。     

Filipin-cholesterol complexes in plasma membranes and cell junctions of Tenebrio molitor epidermis

The polyene antibiotic filipin combines with cholesterol in membranes to form complexes that are readily identifiable in the electron microscope. The distribution of filipin-cholesterol (FC) complexes is most easily studied by freeze-fracture. Larval epidermis of Tenebrio molitor (Insecta, Coleoptera) was maintained in vitro for 48 hr, since the electrophysiological properties of the cells are best characterized under these conditions. The cells were fixed in buffered 3.0% glutaraldehyde at RT for 15 min, transferred to fresh fixative containing 1% DMSO and filipin (final concentration; 0.5 mg/ml) for 3 hr RT. Control cells were treated in fixative containing 1% DMSO only. In freeze fracture replicas, FC complexes appear on the plasma membrane as large circular protrusions measuring 26.5 +/- 6.8 nm (x +/- s.d.) n = 50, in diameter and 17.1 +/- 2.8 nm, n = 50, in height and 11.7 +/- 2.6 nm, n = 25, in depth. Protrusions are about two times more frequent on the E face while pits are several times more frequent on the P face. FC complexes are most abundant (greater than 50/mu m2) on the basal membrane surface of the cells but are excluded from regions of hemidesmosomal plaques that anchor the cells to the basal lamina. FC complexes are also abundant on the apical surfaces of the cells where cuticle secretion occurs. In the lateral regions below the junctional belt, FC complexes are less numerous but often appear to increase in frequency in a graded fashion away from the junctional region. The septate junctions are relatively free of FC complexes except in regions where they open to form islands. These islands often contain gap junctions but the FC complexes rarely invade the particle domains of the gap junctions. Single FC complexes were seen in three out of a total of 97 gap junctions. Exposure of the epidermis to 20-hydroxyecdysone for 24 hr in vitro did not induce the appearance of FC complexes within the cell junctions.     

Anion and cation permeability of a large conductance anion channel in the T84 human colonic cell line

Summary A large conductance multi-state channel was identified and characterized in single channel recordings from cell-attached and excised patches of the human colonic tumor cell line, T84. The channel activity was dependent on the presence of both permeable cations and anions. In Na⁺-free symmetrical Cl^– solutions or Cl^–-free symmetrical Na⁺ solutions the channel was inactive. Addition of 5mm NaCl (Nal or KCl) induced channel activity. The selectivity sequence obtained from the shift in reversal potential was I^–(1.9) > Cl^–(1) > Na⁺(0.5) > K⁺(0.3). SO ₄ ^2– , SCN^– (thiocyanate) and NMDG⁺ were impermeant. Multiple subconductance states were identified at all voltages explored (±90 mV). The minimum conductance encountered in symmetrical 100mm NaCl was a 15 pS substate, the maximum, 210 pS. The channel appeared to be composed of multiples of the 15 pS subunits which were reversibly blocked by the loop diuretic bumetanide (5 m).The authors wish to thank Morris Priddy and Charley Roberson for excellent technical assistance and Linda Pai and Steve Valder for participation in the early experiments. This study was supported by UPSH R01-DK39617 to A. Beaudet. L.V. was supported by a one-year fellowship from the Cystic Fibrosis Foundation.     

The correlation of plasma membrane microvilli and intracellular cyclic AMP content in a rat epitheloid kidney cell line

Modulation of the intracellular concentration of cyclic AMP has been associated with a regulatory role in cell division, cell morphology, and physical properties of the plasma membrane. Untransformed rat kidney cells in culture exhibit epitheloid morphology, high intracellular cyclic AMP levels, and contact inhibition of growth. Untransformed rat kidney cells transformed with the Kirsten murine sarcoma virus exhibit a low cyclic AMP content, rapid growth rate, and a loss of contact inhibition. Scanning electron microscopy reveals a distinctive difference in the surface structure of the two cell types during G1 of the cell cycle. The surface of the transformed cell is covered with microvilli while its untransformed counterpart is devoid of microvilli. The presence of microvilli can be controlled as a function of temperature by two temperature-sensitive mutants of the Kirsten sarcoma virus (ts6t6 and ts371 cl 5). In the ts6t6 mutant, growth at 32°C results in a low cyclic AMP content and the presence of microvilli, while growth at 39°C results in a high cyclic AMP content and a decrease in microvilli. The oposite effect is seen with the ts371 cl 5 mutant. Correlation of cyclic AMP content with the presence of microvilli suggests that this surface phenomenon is a function of cyclic AMP concentration.     

The permeability of reconstituted liposomes containing the purified lens fiber cell integral membrane proteins MP20, MP26 and MP70

Summary A number of lens fiber cell integral membrane proteins have been localized to junctional regions where they have been proposed to play a role in either mediating or controlling cell-to-cell communication. We have examined the effect of three lens fiber cell membrane proteins, MP20, MP26 and MP70, on the permeability properties of unilamellar phospholipid liposomes. This approach has been previously used to examine the channel-forming properties of MP26. Liposome permeability was determined by measuring the effect of Co²⁺ on the quenching of the fluorescence of N-4-nitrobenzo-2-oxa-1,3 diazole phosphatidyl ethanolamine (NBD-PE)-containing liposomes as described previously by Scaglione and Rintoul (Invest. Ophthalmol. Vis. Sci.30:961–966, 1989). The effect of all three proteins on liposome permeability was similar. Permeability was dependent on the protein/phospholipid ratio and was not significantly affected by agents known to modify gap junctional permeability in vivo. Glycophorin A, a non-channel-forming integral membrane protein derived from erythrocytes, was also shown to increase the permeability of unilamellar phospholipid liposomes. The ability of a non-channel membrane protein to increase Co²⁺ quenching of NBD-PE-containing liposomes (presumably in a nonspecific manner) indicates that reports describing the permeability of lens membrane protein-containing liposomes should be interpreted with caution in terms of their relationship to cell-to-cell communication.We would like to thank Dr. Rita Meyer for technical assistance with the freeze-fracture electron microscopy, Drs. Wolfgang Baumann and Barbara Malewicz for the purification of bovine lens lipids, and Dr. Gary Nelsestuen for the use of both the fluorescence and photon correlation spectrophotometers as well as for many helpful discussions. This research was supported by NIH grant EY 05684.