Apical to basolateral surface area ratio and polarity of MDCK cells grown on different supports

The pH-dependence of efflux ratios determined with bidirectional transport assays across cellular monolayers

MDCK/Caco-2 assays serve as essential in vitro tools for evaluating membrane permeability and active transport, especially mediated by P-glycoprotein (P-gp). Despite their utility, challenges remain in quantifying active transport and using the efflux ratio (ER) to determine intrinsic values for active efflux. Such an intrinsic value for P-gp facilitated efflux necessitates knowing whether this transporter transports the neutral or ionic species of a compound. Utilising MDCK-MDR1 assays, we investigate a method for determining transporter substrate fraction preference by studying ER pH-dependence for basic, acidic and non-dissociating compounds. These results are compared with model fits based on various assumptions of transporter species preference. As an unexpected consequence of these assays, we also give evidence for an additional influx transporter at the basolateral membrane, and further extend our model to incorporate this transport. The combined influences of paracellular transport, the previously unaccounted for basolateral influx transporter, as well as potential pH effects on the transporter impedes the extraction of intrinsic values for active transport from the ER. Furthermore, we determined that using inhibitor affects the measurement of paracellular transport. While clear indications of transporter species preference remain elusive, this study enhances understanding of the MDCK system.

Can membrane permeability of zwitterionic compounds be predicted by the solubility-diffusion model?

Zwitterions contain both positively and negatively charged functional groups, resulting in an overall net neutral charge. Nevertheless, the membrane permeability of the zwitterionic form of a compound is assumed to be much lower than the permeability of the uncharged neutral form. Although a significant proportion of pharmaceuticals are zwitterionic, it has not been clear so far whether their permeability is dominated by the permeation of the zwitterionic or the neutral form, since neutral fractions are often quite low as compared to the zwitterionic fraction. This complicates the in silico prediction of the permeability of zwitterionic compounds. In this work, we re-evaluated existing in vitro permeability data from literature measured with Caco-2/MDCK cell assays, using more strict exclusion criteria for effects like diffusion limitation by the aqueous boundary layers, paracellular transport, active transport and retention. Using this re-evaluated data set, we show that extracted intrinsic permeabilities of the neutral fraction are well predicted by the solubility-diffusion model (RMSE = 1.21; n = 18) if the permeability of the zwitterionic species is assumed negligible. Our work thus suggests that only the neutral species is relevant for the membrane permeability of zwitterionic compounds, and that membrane permeability of zwitterionic compounds is indeed predictable by the solubility-diffusion model.

Pitfalls in evaluating permeability experiments with Caco-2/MDCK cell monolayers

When studying the transport of molecules across biological membranes, intrinsic membrane permeability (P0) is more informative than apparent permeability (Papp), because it eliminates external (setup-specific) factors, provides consistency across experiments and mechanistic insight. It is thus an important building block for modeling the total permeability in any given scenario. However, extracting P0 is often difficult, if not impossible, when the membrane is not the dominant transport resistance. In this work, we set out to analyze Papp values measured with Caco-2/MDCK cell monolayers of 69 literature references. We checked the Papp values for a total of 318 different compounds for the extractability of P0, considering possible limitations by aqueous boundary layers, paracellular transport, recovery issues, active transport, a possible proton flux limitation, and sink conditions. Overall, we were able to extract 77 reliable P0 values, which corresponds to about one quarter of the total compounds analyzed, while about half were limited by the diffusion through the aqueous layers. Compared to an existing data set of P0 values published by Avdeef, our approach resulted in a much higher exclusion of compounds. This is a consequence of stricter compound- and reference-specific exclusion criteria, but also because we considered possible concentration-shift effects due to different pH values in the aqueous layers, an effect only recently described in literature. We thus provide a consistent and reliable set of P0, e.g. as a basis for future modeling.

Epithelial cells sacrifice excess area to preserve fluidity in response to external mechanical stress

Viscoelastic properties of epithelial cells subject to shape changes were monitored by indentation-retraction/relaxation experiments. MDCK II cells cultured on extensible polydimethylsiloxane substrates were laterally stretched and, in response, displayed increased cortex contractility and loss of excess surface area. Thereby, the cells preserve their fluidity but inevitably become stiffer. We found similar behavior in demixed cell monolayers of ZO-1/2 double knock down (dKD) cells, cells exposed to different temperatures and after removal of cholesterol from the plasma membrane. Conversely, the mechanical response of single cells adhered onto differently sized patches displays no visible rheological change. Sacrificing excess surface area allows the cells to respond to mechanical challenges without losing their ability to flow. They gain a new degree of freedom that permits resolving the interdependence of fluidity β on stiffness [Formula: see text]. We also propose a model that permits to tell apart contributions from excess membrane area and excess cell surface area.

Viscoelasticity of basal plasma membranes and cortices derived from MDCK II cells

The mechanical properties of cells are largely determined by the architecture and dynamics of their viscoelastic cortex, which consists of a contractile, cross-linked actin mesh attached to the plasma membrane via linker proteins. Measuring the mechanical properties of adherent, polarized epithelial cells is usually limited to the upper, i.e., apical side, of the cells because of their accessibility on culture dishes. Therefore, less is known about the viscoelastic properties of basal membranes. Here, I investigate the viscoelastic properties of basolateral membranes derived from polarized MDCK II epithelia in response to external deformation and compare them to living cells probed at the apical side. MDCK II cells were grown on porous surfaces to confluence, and the upper cell body was removed via a squirting-lysis protocol. The free-standing, defoliated basal membranes were subject to force indentation and relaxation experiments permitting a precise assessment of cortical viscoelasticity. A new theoretical framework to describe the force cycles is developed and applied to obtain the time-dependent area compressibility modulus of cell cortices from adherent cells. Compared with the viscoelastic response of living cells, the basolateral membranes are substantially less fluid and stiffer but obey to the same universal scaling law if excess area is taken correctly into account.

Successful Prediction of Human Steady-State Unbound Brain-to-Plasma Concentration Ratio of P-gp Substrates Using the Proteomics-Informed Relative Expression Factor Approach

In order to optimize central nervous system (CNS) drug development, accurate prediction of the drug's human steady-state unbound brain interstitial fluid-to-plasma concentration ratio (K_p,uu,brain ) is critical, especially for drugs that are effluxed by the multiple drug resistance transporters (e.g., P-glycoprotein, P-gp). Due to lack of good in vitro human blood-brain barrier models, we and others have advocated the use of a proteomics-informed relative expressive factor (REF) approach to predict K_p,uu,brain . Therefore, we tested the success of this approach in humans, with a focus on P-gp substrates, using brain positron emission tomography imaging data for verification. To do so, the efflux ratio (ER) of verapamil, N-desmethyl loperamide, and metoclopramide was determined in human P-gp-transfected MDCKII cells using the Transwell assay. Then, using the ER estimate, K_p,uu,brain of the drug was predicted using REF (ER approach). Alternatively, in vitro passive and P-gp-mediated intrinsic clearances (CLs) of these drugs, estimated using a five-compartmental model, were extrapolated to in vivo using REF (active CL) and brain microvascular endothelial cells protein content (passive CL). The ER approach successfully predicted K_p,uu,brain of all three drugs within twofold of observed data and within 95% confidence interval of the observed data for verapamil and N-desmethyl loperamide. Using the in vitro-to-in vivo extrapolated clearance approach, K_p,uu,brain was reasonably well predicted but not the brain unbound interstitial fluid drug concentration-time profile. Therefore, we propose that the ER approach be used to predict K_p,uu,brain of CNS candidate drugs to enhance their success in development.

Investigation on the suitability of milk-derived primary bovine mammary epithelial cells grown on permeable membrane supports as an in vitro model for lactation

This study aimed to establish an in vitro model for lipid synthesis in primary bovine mammary epithelial cells (pbMECs) extracted from milk and cultured on Transwell permeable supports (TW culture). The suitability of these cells as a functional model for lactation was assessed by measuring κ-casein (CSN3) and diacylglycerol acyl transferase 1 (DGAT1) gene expression, the presence of intracellular lipid droplets, and the concentration of triacylglycerol in the cell lysates. The functionality of the milk-derived pbMECs cultured under lactogenic conditions, with and without oleic acid supplementation, was evaluated by comparing the cells grown on Transwell supports to cells grown on an extracellular matrix (ECM) gel (3D culture) or a plastic surface (2D culture). Furthermore, the functionality of milk-derived cells was compared to pbMECs obtained from bovine mammary tissue. Here, we show that in both tissue and milk-derived pbMECs, 3D culture offered the most suitable in vitro environment and led to increased levels of CSN3 and DGAT1 gene expression along with increased intracellular triacylglycerol content. The TW culture conditions also resulted in increased DGAT1 gene expression compared to the 2D conditions and milk-derived pbMECs cultured on TW inserts showed the highest viability compared to cells grown under 2D or 3D treatments. However, this was not observed for tissue-derived pbMECs, suggesting that TW culture may offer a beneficial environment specifically for milk-derived cells. We suggest that with further optimization of the culture conditions, TW culture may present a suitable model for the study of milk lipid synthesis in pbMECs.

Polarised epithelial monolayers of the gastric mucosa reveal insights into mucosal homeostasis and defence against infection

OBJECTIVE

Helicobacter pylori causes life-long colonisation of the gastric mucosa, leading to chronic inflammation with increased risk of gastric cancer. Research on the pathogenesis of this infection would strongly benefit from an authentic human in vitro model.

DESIGN

Antrum-derived gastric glands from surgery specimens served to establish polarised epithelial monolayers via a transient air-liquid interface culture stage to study cross-talk with H. pylori and the adjacent stroma.

RESULTS

The resulting 'mucosoid cultures', so named because they recapitulate key characteristics of the gastric mucosa, represent normal stem cell-driven cultures that can be passaged for months. These highly polarised columnar epithelial layers encompass the various gastric antral cell types and secrete mucus at the apical surface. By default, they differentiate towards a foveolar, MUC5AC-producing phenotype, whereas Wnt signalling stimulates proliferation of MUC6-producing cells and preserves stemness-reminiscent of the gland base. Stromal cells from the lamina propria secrete Wnt inhibitors, antagonising stem-cell niche signalling and inducing differentiation. On infection with H. pylori, a strong inflammatory response is induced preferentially in the undifferentiated basal cell phenotype. Infection of cultures for several weeks produces foci of viable bacteria and a persistent inflammatory condition, while the secreted mucus establishes a barrier that only few bacteria manage to overcome.

CONCLUSION

Gastric mucosoid cultures faithfully reproduce the features of normal human gastric epithelium, enabling new approaches for investigating the interaction of H. pylori with the epithelial surface and the cross-talk with the basolateral stromal compartment. Our observations provide striking insights in the regulatory circuits of inflammation and defence.

Independent impedimetric analysis of two cell populations co-cultured on opposite sides of a porous support

The transepithelial or -endothelial electrical resistance (TEER) is a very common and routinely recorded parameter describing the expression of barrier-forming cell-cell contacts (tight junctions) in quantitative terms. To determine TEER, barrier-forming cell monolayers are cultured on porous filter supports that separate two fluid compartments. The frequency-dependent impedance of the cell layer is then recorded and analyzed by means of equivalent circuit modelling providing TEER and the cell layer capacitance. The latter serves as a quantitative indicator for membrane topography. When cells are co-cultured on opposite sides of such a porous support to model more complex biological barriers, TEER readings will integrate over both cell layers and the individual contributions are not assessable. This study describes the modification of commonly used porous filter inserts by coating their backside with a thin gold-film. When this gold-film is used as an additional electrode, both cell layers can be studied separately by impedance analysis. The electrical parameters of either cell layer are assessable independently by switching between different electrode combinations. The performance of this new approach is illustrated and documented by experiments that (i) follow the de novo formation of cell junctions between initially suspended cells and (ii) the manipulation of mature cell-cell junctions by cytoskeleton-active drugs. Both assays confirm that both cell layers are monitored entirely independently.

Extrapolation of Elementary Rate Constants of P-glycoprotein-Mediated Transport from MDCKII-hMDR1-NKI to Caco-2 Cells

The best parameters for incorporation into mechanistic physiologically based pharmacokinetic models for transporters are system-independent kinetic parameters and active (not total) transporter levels. Previously, we determined the elementary rate constants for P-glycoprotein (P-gp)-mediated transport (on- and off-rate constants from membrane to P-gp binding pocket and efflux rate constant into the apical chamber) using the structural mass action kinetic model in confluent MDCKII-hMDR1-NKI cell monolayers. In the present work, we extended the kinetic analysis to Caco-2 cells for the first time and showed that the elementary rate constants are very similar compared with MDCKII-hMDR1-NKI cells, suggesting they primarily depend on the interaction of the compound with P-gp and are therefore mostly independent of the in vitro system used. The level of efflux active (not total) P-gp is also fitted by our model. The estimated level of efflux active P-gp was 5.0 ± 1.4-fold lower in Caco-2 cells than in MDCKII-hMDR1-NKI cells. We also kinetically identified the involvement of a basolateral uptake transporter for both digoxin and loperamide in Caco-2 cells, as found previously in MDCKII-hMDR1-NKI cells, due to their low passive permeability. This demonstrates the value of our P-gp structural model as a diagnostic tool in detecting the importance of other transporters, which cannot be unambiguously done by the Michaelis-Menten approach. The system-independent elementary rate constants for P-gp obtained in vitro are more fundamental parameters than those obtained using Michaelis-Menten steady-state equations. This suggests they will be more robust mechanistic parameters for incorporation into physiologically based pharmacokinetic models for transporters.

Microvilli Morphology Can Affect Efflux Active P-Glycoprotein in Confluent MDCKII -hMDR1-NKI and Caco-2 Cell Monolayers

From fits of drug transport kinetics across confluent MDCKII-hMDR1-NKI and Caco-2 cell monolayers we estimated the levels of efflux active P-glycoprotein (P-gp) in these two cell lines (companion paper). In the present work, we compared the efflux active P-gp number to the total P-gp level, using liquid chromatography-tandem mass spectrometry, and showed that in Caco-2 cells total P-gp is about 10-fold greater than efflux active P-gp, whereas in MDCKII-hMDR1-NKI cells these values are within twofold. We further visualized the microvilli in MDCKII-hMDR1-NKI and Caco-2 cells using three-dimensional structured illumination super-resolution microscopy and found that the microvilli in Caco-2 cells are taller and more densely packed than those in MDCK-hMDR1-NKI cells. We hypothesized over 10 years ago that only P-gp at the tips of the microvilli contribute significantly to efflux activity, whereas the remaining P-gp are involved in a futile cycle of efflux of amphipathic drugs from the microvillus membrane, followed by their reabsorption into the same or nearby microvillous membranes. The difference between the levels of total and efflux active P-gp in Caco-2 cells can be explained by the more densely packed microvilli in Caco-2 cells, which would lead to a substantial fraction of P-gp not contributing to final release of drug into the apical chamber. Our results suggest that the effect of microvilli morphology differences between in vitro and in vivo systems must be considered when scaling transporter activity for efflux transporters of amphipathic compounds, for example, P-gp.

Elastic properties of epithelial cells probed by atomic force microscopy

Cellular mechanics plays a crucial role in many biological processes such as cell migration, cell growth, embryogenesis, and oncogenesis. Epithelia respond to environmental cues comprising biochemical and physical stimuli through defined changes in cell elasticity. For instance, cells can differentiate between certain properties such as viscoelasticity or topography of substrates by adapting their own elasticity and shape. A living cell is a complex viscoelastic body that not only exhibits a shell architecture composed of a membrane attached to a cytoskeleton cortex but also generates contractile forces through its actomyosin network. Here we review cellular mechanics of single cells in the context of epithelial cell layers responding to chemical and physical stimuli. This article is part of a Special Issue entitled: Mechanobiology.

Compartmental models for apical efflux by P-glycoprotein--part 1: evaluation of model complexity

PURPOSE

With the goal of quantifying P-gp transport kinetics, Part 1 of these manuscripts evaluates different compartmental models and Part 2 applies these models to kinetic data.

METHODS

Models were developed to simulate the effect of apical efflux transporters on intracellular concentrations of six drugs. The effect of experimental variability on model predictions was evaluated. Several models were evaluated, and characteristics including membrane configuration, lipid content, and apical surface area (asa) were varied.

RESULTS

Passive permeabilities from MDCK-MDR1 cells in the presence of cyclosporine gave lower model errors than from MDCK control cells. Consistent with the results in Part 2, model configuration had little impact on calculated model errors. The 5-compartment model was the simplest model that reproduced experimental lag times. Lipid content and asa had minimal effect on model errors, predicted lag times, and intracellular concentrations. Including endogenous basolateral uptake activity can decrease model errors. Models with and without explicit membrane barriers differed markedly in their predicted intracellular concentrations for basolateral drug exposure. Single point data resulted in clearances similar to time course data.

CONCLUSIONS

Compartmental models are useful to evaluate the impact of efflux transporters on intracellular concentrations. Whereas a 3-compartment model may be sufficient to predict the impact of transporters that efflux drugs from the cell, a 5-compartment model with explicit membranes may be required to predict intracellular concentrations when efflux occurs from the membrane. More complex models including additional compartments may be unnecessary.

Fluorescein leakage test: a useful tool in ocular safety assessment

The fluorescein leakage test (FLT) provides information on the effects of xenobiotics on the impermeability (gate function) of epithelial cell monolayers, and their recovery after exposure. The aim of this study was to assess the validity of this test in the ocular safety assessment of surfactant-based products with various irritant potencies. Madin-Darby canine kidney cells were grown to confluency on microporous membranes and exposed for 15 min to increasing concentrations of test substances. Damage was evaluated by measuring the amount of Na-fluorescein that passed through the monolayer in 30 min, starting just after exposure. Recovery was assessed 4, 24, 48 and 72 hr later. For each sample and each time point, the amounts of test substance that produced 10% and 20% leakage (FL(10) and FL(20)) compared with a cell-free control were calculated. For the 43 samples, FL(20) values ranged from 0.65 to 1000 mg/ml. These values increased or decreased with time according to the substance. In particular, cell monolayers showed very different recoveries after exposure to anionic and cationic substances with similar initial FL(20) values. These in vitro data correlated well with historical Draize in vivo test data (Spearman's varrho > 0.90). The FLT is therefore useful as a complement to other in vitro methods for the ocular safety evaluation of cosmetics.

Evaluation of tissue culture insert membrane compatibility in the fluorescein leakage assay

The Fluorescein Leakage (FL) test, a short-term in vitro assay measuring damage on exposure to eye irritants to a transepithelial barrier permeability system, has been assessed as an alternative to the Draize rabbit eye irritation assay in the EU/Home Office international validation trial. The assay is based on the degree of leakage of sodium fluorescein through a confluent layer of Madin-Darby canine kidney epithelial (MDCK) cells grown on a tissue culture insert following exposure to a potential irritant. Several different types of tissue culture inserts with different membrane growth surfaces, pore sizes and pore densities have been employed in the past. Problems with chemical binding and other incompatibilities with insert membranes can affect the sensitivity of MDCK cells. Cationic surfactants in particular appear to interfere with the passage of fluorescein, leading to underestimation of damage to MDCK cellular function. A blind comparison between MDCK cells cultured on Anopore() and the Millicell-HA inserts using the EU/Home Office set of surfactants indicate that the sensitivity of cells cultured on Anopore inserts appeared to correlate more closely with in vivo data. The ability to microscopically evaluate the status of MDCK cultures through Anopore further recommends this membrane as possibly the better choice for the FL assay.

Expression Profile of Drug and Nutrient Absorption Related Genes in Madin-Darby Canine Kidney (MDCK) Cells Grown under Differentiation Conditions

The expression levels of genes involved in drug and nutrient absorption were evaluated in the Madin-Darby Canine Kidney (MDCK) in vitro drug absorption model. MDCK cells were grown on plastic surfaces (for 3 days) or on Transwell^® membranes (for 3, 5, 7, and 9 days). The expression profile of genes including ABC transporters, SLC transporters, and cytochrome P450 (CYP) enzymes was determined using the Affymetrix^® Canine GeneChip^®. Expression of genes whose probe sets passed a stringent confirmation process was examined. Expression of a few transporter (MDR1, PEPT1 and PEPT2) genes in MDCK cells was confirmed by RT-PCR. The overall gene expression profile was strongly influenced by the type of support the cells were grown on. After 3 days of growth, expression of 28% of the genes was statistically different (1.5-fold cutoff, p < 0.05) between the cells grown on plastic and Transwell^® membranes. When cells were differentiated on Transwell^® membranes, large changes in gene expression profile were observed during the early stages, which then stabilized after 5–7 days. Only a small number of genes encoding drug absorption related SLC, ABC, and CYP were detected in MDCK cells, and most of them exhibited low hybridization signals. Results from this study provide valuable reference information on endogenous gene expression in MDCK cells that could assist in design of drug-transporter and/or drug-enzyme interaction studies, and help interpret the contributions of various transporters and metabolic enzymes in studies with MDCK cells.

In vitro surfactant structure-toxicity relationships: implications for surfactant use in sexually transmitted infection prophylaxis and contraception

BACKGROUND

The need for woman-controlled, cheap, safe, effective, easy-to-use and easy-to-store topical applications for prophylaxis against sexually transmitted infections (STIs) makes surfactant-containing formulations an interesting option that requires a more fundamental knowledge concerning surfactant toxicology and structure-activity relationships.

METHODOLOGY/PRINCIPAL FINDINGS

We report in vitro effects of surfactant concentration, exposure time and structure on the viability of mammalian cell types typically encountered in the vagina, namely, fully polarized and confluent epithelial cells, confluent but non-polarized epithelial-like cells, dendritic cells, and human sperm. Representatives of the different families of commercially available surfactants--nonionic (Triton X-100 and monolaurin), zwitterionic (DDPS), anionic (SDS), and cationic (C(n)TAB (n = 10 to 16), C(12)PB, and C(12)BZK)--were examined. Triton X-100, monolaurin, DDPS and SDS were toxic to all cell types at concentrations around their critical micelle concentration (CMC) suggesting a non-selective mode of action involving cell membrane destabilization and/or destruction. All cationic surfactants were toxic at concentrations far below their CMC and showed significant differences in their toxicity toward polarized as compared with non-polarized cells. Their toxicity was also dependent on the chemical nature of the polar head group. Our results suggest an intracellular locus of action for cationic surfactants and show that their structure-activity relationships could be profitably exploited for STI prophylaxis in vaginal gel formulations. The therapeutic indices comparing polarized epithelial cell toxicity to sperm toxicity for all surfactants examined, except C(12)PB and C(12)BZK, does not justify their use as contraceptive agents. C(12)PB and C(12)BZK are shown to have a narrow therapeutic index recommending caution in their use in contraceptive formulations.

CONCLUSIONS/SIGNIFICANCE

Our results contribute to understanding the mechanisms involved in surfactant toxicity, have a predictive value with regard to their safety, and may be used to design more effective and less harmful surfactants for use in topical applications for STI prophylaxis.

A plate reader-based method for cell water permeability measurement

Cell volume and water permeability measurements in cultured mammalian cells are typically conducted under a light microscope. Many of the employed approaches are time consuming and not applicable to a study of confluent epithelial cell monolayers. We present here an adaptation of a calcein-quenching-based approach for a plate reader. A standard curve of fluorescence intensities at equilibrium has been recorded, following a shift from 285 mosmol/kgH2O to a series of altered extracellular osmolyte concentrations, ranging from final concentrations of 185 to 585 mosmol/kgH2O, by changing buffer d-mannitol concentrations. Similarly, according average cell volumes have been measured in suspension in a Coulter counter (particle-sizing device). Based on these measurements, we have derived an equation that facilitates the modeling of cell volume changes based on fluorescence intensity changes. We have utilized the method to study the role of a carboxyl-terminus aquaporin (AQP)-2 phosphorylation site, which is known to affect AQP2 membrane trafficking, in heterologous type I Madin-Darby canine kidney cells. We find that water permeability in cells expressing phosphorylation site mutants was in the following order: AQP2-S256D > AQP2 wild-type > AQP2-S256A. We propose that the method can be applied to study AQP function and more generally to study cell volume changes in adherent cell lines. Furthermore, it should be adaptable for AQP inhibitor screening in chemical compound libraries.

Synthesis, trafficking, and localization of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors are members of the G-protein coupled receptor superfamily that are expressed in and regulate the function of neurons, cardiac and smooth muscle, glands, and many other cell types and tissues. The correct trafficking of membrane proteins to the cell surface and their subsequent localization at appropriate sites in polarized cells are required for normal cellular signaling and physiological responses. This review will summarize work on the synthesis and trafficking of muscarinic receptors to the plasma membrane and their localization at the cell surface.

Variability in Caco-2 and MDCK Cell-Based Intestinal Permeability Assays

Models predictive of intestinal drug absorption are important in drug development to identify compounds with promising biopharmaceutical properties. Since permeability is a factor affecting absorption, cell culture models (e.g., Caco-2, MDCK) have been developed to predict drug transport from the intestinal lumen into the bloodstream. The differences as to how the assays are performed, along with heterogeneity of the cell lines, have lead to different permeability values for the same drug. Transport and metabolic properties of cultured cells can vary due to culture conditions, seeding density, passage number, confluency, filter support, monolayer age, and stage of differentiation. During the transport experiment, cell absorption properties can change due to the composition and pH of the transport buffer, solute concentration and solubility, temperature, additives and/or cosolvents, agitation, sampling schedule, sink conditions, and analytical methods. Such variability within a laboratory can be avoided by characterizing a cell culture method and setting acceptance criteria in terms of monolayer integrity, passive transport, and active transport. The repeated evaluation of reference compounds will then facilitate intra-laboratory comparisons.

The third dimension bridges the gap between cell culture and live tissue

Moving from cell monolayers to three-dimensional (3D) cultures is motivated by the need to work with cellular models that mimic the functions of living tissues. Essential cellular functions that are present in tissues are missed by 'petri dish'-based cell cultures. This limits their potential to predict the cellular responses of real organisms. However, establishing 3D cultures as a mainstream approach requires the development of standard protocols, new cell lines and quantitative analysis methods, which include well-suited three-dimensional imaging techniques. We believe that 3D cultures will have a strong impact on drug screening and will also decrease the use of laboratory animals, for example, in the context of toxicity assays.

Phase coexistence and connectivity in the apical membrane of polarized epithelial cells

Although it is well described in model membranes, little is known about phase separation in biological membranes. Here, we provide evidence for a coexistence of at least two different lipid bilayer phases in the apical plasma membrane of epithelial cells. Phase connectivity was assessed by measuring long-range diffusion of several membrane proteins by fluorescence recovery after photobleaching in two polarized epithelial cell lines and one fibroblast cell line. In contrast to the fibroblast plasma membrane, in which all of the proteins diffused with similar characteristics, in the apical membrane of epithelial cells the proteins could be divided into two groups according to their diffusion characteristics. At room temperature ( approximately 25 degrees C), one group showed fast diffusion and complete recovery. The other diffused three to four times slower and, more importantly, displayed only partial recovery. Only the first group comprises proteins that are believed to be associated with lipid rafts. The partial recovery is not caused by topological constraints (microvilli, etc.), cytoskeletal constraints, or protein-protein interactions, because all proteins show 100% recovery in fluorescence recovery after photobleaching experiments at 37 degrees C. In addition, the raft-associated proteins cannot be coclustered by antibodies on the apical membrane at 12 degrees C. The interpretation that best fits these data is that the apical membrane of epithelial cells is a phase-separated system with a continuous (percolating) raft phase <25 degrees C in which isolated domains of the nonraft phase are dispersed, whereas at 37 degrees C the nonraft phase becomes the continuous phase with isolated domains of the raft phase dispersed in it.

The Steady-State Michaelis–Menten Analysis of P-Glycoprotein Mediated Transport Through a Confluent Cell Monolayer Cannot Predict the Correct Michaelis Constant Km

PURPOSE

Typically, the kinetics of membrane transport is analyzed using the steady-state Michaelis-Menten (or Eadie-Hofstee or Hanes) equations. This approach has been successful when the substrate is picked up from the aqueous phase, like a water-soluble enzyme, for which the Michaelis-Menten steady-state analysis was developed. For membrane transporters whose substrate resides in the lipid bilayer of the plasma membrane, like P-glycoprotein (P-gp), there has been no validation of the accuracy of the steady-state analysis because the elementary rate constants for transport were not known.

METHODS

Recently, we fitted the mass action elementary kinetic rate constants of P-gp transport of three different drugs through a confluent monolayer of MDCKII-hMDR1 cells. With these elementary rate constants in hand, we use computer simulations to assess the accuracy of the steady-state Michaelis-Menten parameters. This limits the simulation to parameter ranges known to be physiologically relevant.

RESULTS

Using over 2,300 different vectors of initial elementary parameters spanning the space bounded by the three drugs, which defines 2,300 "virtual substrates", the concentrations of substrate transported were calculated and fitted to Eadie-Hofstee plots. Acceptable plots were obtained for 1,338 cases.

CONCLUSION

The fitted steady-state Vmax values from the analysis correlated to within a factor of 2-3 with the values predicted from the elementary parameters. However, the fitted Km value could be generated by a wide range of underlying "molecular" Km values. This is because of the convolution of the drug passive permeability kinetics into the fitted Km. This implies that Km values measured in simpler systems, e.g., microsomes or proteoliposomes, even if accurate, would not predict the Km values for the confluent monolayer system or, by logical extension, in vivo. Reliable in vitro-in vivo extrapolation seems to require using the elementary rate constants rather than the Michaelis-Menten steady-state parameters.

The elementary mass action rate constants of P-gp transport for a confluent monolayer of MDCKII-hMDR1 cells

The human multi-drug resistance membrane transporter, P-glycoprotein, or P-gp, has been extensively studied due to its importance to human health and disease. Thus far, the kinetic analysis of P-gp transport has been limited to steady-state Michaelis-Menten approaches or to compartmental models, neither of which can prove molecular mechanisms. Determination of the elementary kinetic rate constants of transport will be essential to understanding how P-gp works. The experimental system we use is a confluent monolayer of MDCKII-hMDR1 cells that overexpress P-gp. It is a physiologically relevant model system, and transport is measured without biochemical manipulations of P-gp. The Michaelis-Menten mass action reaction is used to model P-gp transport. Without imposing the steady-state assumptions, this reaction depends upon several parameters that must be simultaneously fitted. An exhaustive fitting of transport data to find all possible parameter vectors that best fit the data was accomplished with a reasonable computation time using a hierarchical algorithm. For three P-gp substrates (amprenavir, loperamide, and quinidine), we have successfully fitted the elementary rate constants, i.e., drug association to P-gp from the apical membrane inner monolayer, drug dissociation back into the apical membrane inner monolayer, and drug efflux from P-gp into the apical chamber, as well as the density of efflux active P-gp. All three drugs had overlapping ranges for the efflux active P-gp, which was a benchmark for the validity of the fitting process. One novel finding was that the association to P-gp appears to be rate-limited solely by drug lateral diffusion within the inner monolayer of the plasma membrane for all three drugs. This would be expected if P-gp structure were open to the lipids of the apical membrane inner monolayer, as has been suggested by recent structural studies. The fitted kinetic parameters show how P-gp efflux of a wide range of xenobiotics has been maximized.

The Effect of Cell Culture Conditions on Saquinavir Transport Through, and Interactions with, MDCKII Cells Overexpressing hMDR1

MDCK cells are cultured using wide-ranging conditions and can produce variable results. To develop a standard protocol for studying saquinavir transport using MDCKII cells, stably transfected MDCKII cells overexpressing human Pgp (MDCKII-PGP) and MDCKII wild-type cells (MDCKII/wt) were used to evaluate the combined effects of seeding density (6.9 x 10(5) or 5 x 10(4) cells/cm2), substratum (polycarbonate +/- collagen coating) and saquinavir presence on monolayer integrity, Pgp expression, and saquinavir transport. The saquinavir efflux ratio (ratio of BL --> AP/AP --> BL permeability) for MDCKII-PGP cells (6.9 x 10(5) cells/cm2) was 57 with variable mannitol permeabilities. Consistent mannitol permeabilities and higher saquinavir efflux ratios were obtained with 5 x 10(4) cells/cm2 on polycarbonate (78) or collagen-coated polycarbonate (126). The MDCKII/wt saquinavir efflux ratio was 9. Saquinavir presence increased paracellular permeability for all treatments relative to cells seeded onto collagen-coated membranes. Collagen coating caused increased Pgp expression and saquinavir efflux ratios correlated (r2 = 0.96) with Pgp expression levels [MDCKII-PGP (on collagen-coated polycarbonate) > MDCKII-PGP (on polycarbonate) > MDCKII/wt (on collagen-coated polycarbonate)]. These results directly and quantitatively link interrelated differences in cell culture conditions to changes in monolayer integrity, transporter expression, and active transport; and emphasize the critical application of controls in cell culture models.

Transcytosis: crossing cellular barriers

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Functional characterization of monocarboxylic acid, large neutral amino acid, bile acid and peptide transporters, and P-glycoprotein in MDCK and Caco-2 cells

Bidirectional transport studies were conducted to determine whether substrates of five intestinal transporters showed carrier-mediated asymmetric transport across MDCK (Madin-Darby canine kidney) cell monolayers grown under standard conditions. Drug concentrations were quantitated using liquid scintillation counting, liquid chromatography/mass spectrometry/mass spectrometry, or liquid chromatography/mass spectrometry. In the presence of a pH gradient, benzoic acid exhibited net apical-to-basolateral transport, with apparent permeability ratios (apical-to-basolateral permeability/basolateral-to-apical permeability) ranging from 14 to 25. The addition of valproic acid reduced the permeability ratio by 70-90%. Cephalexin transport also exhibited net absorption in the presence of a pH gradient, with apparent permeability ratios ranging from 14 to 71, depending on growth conditions. Radiolabeled phenylalanine exhibited a low level of carrier-mediated absorption with an apparent permeability ratio of 1.8 that was reduced to 1.0 in the presence of unlabeled L-phenylalanine. Taurocholic acid did not exhibit carrier-mediated absorption. Cyclosporine and fexofenadine exhibited P-glycoprotein-mediated efflux from both MDCK and Caco-2 cells, which was more sensitive to inhibition in MDCK cells. These results suggest that although MDCK cell monolayers may be a useful model for evaluating transport by the absorptive monocarboxylic acid and peptide transporters and the efflux transporter, P-glycoprotein, they are not useful for predicting large neutral amino acid or bile acid transport in the intestine.

CEACAM1 isoforms with different cytoplasmic domains show different localization, organization and adhesive properties in polarized epithelial cells

CEACAM1 is a signaling cell adhesion molecule expressed in epithelia,vessel endothelia and leukocytes. It is expressed as two major isoforms with different cytoplasmic domains. CEACAM1 occurs both in cell-cell contact areas and on apical surfaces of polarized epithelial cells, but it is not known how the different isoforms are distributed in polarized cells or what the functions of CEACAM1 are in the apical surfaces. We investigated the localization and organization of the two CEACAM1 isoforms in transfected,polarized MDCK cells by confocal microscopy and differential surface labelling. CEACAM1-L was found on both the apical and the lateral surfaces,whereas CEACAM1-S appeared exclusively on the apical surfaces. Maintenance of the lateral localization of CEACAM1-L required homophilic binding between CEACAM1-L molecules on adjacent cells. Double-labelling with anti-CEACAM1 antibodies directed against different epitopes indicated that apical CEACAM1-L occurred either in a homophilic adhesive state or in a free non-adhesive state. CEACAM1-S appeared almost exclusively in the homophilic adhesive state. These findings suggest that CEACAM1 mediates adhesive bonds between adjacent microvilli on the apical surfaces.

Stimulation of epithelial tissue migration by certain porous topographies is independent of fluid flux

A surface with columnar pores 0.1 or 0.4 microm in diameter is shown to have a novel effect on the migration of corneal epithelial tissue sheets; migration is stimulated in a nondirectional manner with respect to migration over a planar, nonporous surface (Dalton, Evans, McFarland, and Steele, J Biomed Mater Res 1999;45:384-394; Steele, Johnson, McLean, Beumer, and Griesser, J Biomed Mater Res 2000;50:475-482). By blind-ending the pores, we show that this increase in tissue migration is not dependent on fluid flux through the pores and so appears to occur as a result of surface topography. From transmission electron micrographs, the migrating tissue appears to form either close contacts or focal adhesions at the edge of some pore channels; we speculate that this may provide a fulcrum for the enhanced migration. Scanning electron micrographs suggest that within tissue that migrates over the surfaces that contain blind-ended pores, the cells are more extensively spread than those in tissue migrating on a planar surface. The migration of disaggregated epithelial cells is enhanced on surfaces that contain 0.1- or 0.4-microm-diameter pores (compared with a planar surface), and this is similarly independent of fluid flux.

Localization of GFP-tagged concentrative nucleoside transporters in a renal polarized epithelial cell line

Many nucleosides undergo active reabsorption within the kidney, probably via nucleoside transporters. To date, two concentrative nucleoside transporters have been cloned, the sodium-dependent purine-selective nucleoside transporter (SPNT) and concentrative nucleoside transporter 1 (CNT1). We report the stable expression of green fluorescence protein (GFP)-tagged SPNT and CNT1 in Madin-Darby canine kidney (MDCK) cells, a polarized renal epithelial line. We demonstrate that the GFP tag does not alter the substrate selectivity and only modestly affects the kinetic activity of the transporters. By using confocal microscopy and functional studies, both SPNT and CNT1 are localized primarily to the apical membrane of MDCK and LLC-PK(1) cells. Apical localization of these transporters suggests a role in renal nucleoside reabsorption and regulation of tubular function via the adenosine pathway.

Tryptophan- and dileucine-based endocytosis signals in the neonatal Fc receptor

The neonatal Fc receptor, FcRn, transports immunoglobulin G across intestinal cells in suckling rats. FcRn enters these cells by endocytosis and is present on the apical and basolateral surfaces. We investigated the roles of aromatic amino acids and a dileucine motif in the cytoplasmic domain of rat FcRn. We expressed mutant FcRn in which alanine replaced Trp-311, Leu-322, and Leu-323, or Phe-340 in the inner medullary collecting duct cell line IMCD. Individual replacement of the aromatic amino acids or the dileucine motif only partially blocked endocytosis of (125)I-Fc, whereas uptake by FcRn containing alanine residues in place of both Trp-311 and the dileucine motif was reduced to the level obtained with the tailless receptor. Leu-314 was required for the function of the tryptophan-based endocytosis signal, and Asp-317 and Asp-318 were required for the dileucine-based signal. Nonvectorial delivery of newly synthesized FcRn to the two cell surfaces was unaffected by loss of the endocytosis signals. However, the steady-state distribution of endocytosis mutants was predominantly apical, unlike wild-type FcRn, which was predominantly basolateral. This shift appeared to arise because the loss of endocytosis signals inhibited apical to basolateral transcytosis of FcRn more than basolateral to apical transcytosis.

Asymmetric distribution of muscarinic acetylcholine receptors in Madin-Darby canine kidney cells

We have characterized the muscarinic ACh receptors (mAChRs) expressed in Madin- Darby canine kidney (MDCK) strain II epithelial cells. Binding studies with the membrane-impermeable antagonist N-[(3)H]methylscopolamine demonstrated that mAChRs are approximately 2.5 times more abundant on the basolateral than on the apical surface. Apical, but not basolateral, mAChRs inhibited forskolin-stimulated adenylyl cyclase activity in response to the agonist carbachol. Neither apical nor basolateral mAChRs exhibited detectable carbachol-stimulated phospholipase C activity. Carbachol application to the apical or the basolateral membrane resulted in a threefold increase in intracellular Ca(2+) concentration, which was completely inhibited by pertussis toxin on the apical side and partially inhibited on the basolateral side. RT-PCR analysis showed that MDCK cells express the M(4) and M(5) receptor mRNAs. These data suggest that M(4) receptors reside on the apical and basolateral membranes of polarized MDCK strain II cells and that the M(5) receptor may reside in the basolateral membrane of a subset of cells.

Isoforms of the Lutheran/basal cell adhesion molecule glycoprotein are differentially delivered in polarized epithelial cells. Mapping of the basolateral sorting signal to a cytoplasmic di-leucine motif

Lu and Lu(v13) are two glycoprotein (gp) isoforms that belong to the immunoglobulin superfamily and carry both the Lutheran (Lu) blood group antigens and the basal cell adhesion molecule epithelial cancer antigen. Lu (85 kDa) and Lu(v13) (78 kDa) gps, which differ only in the length of their cytoplasmic domain, are adhesion molecules that bind laminin. In nonerythroid tissues, the Lu/basal cell adhesion molecule antigens are predominantly expressed in the endothelium of blood vessel walls and in the basement membrane region of normal epithelial cells, whereas they exhibit a nonpolarized expression in some epithelial cancers. Here, we analyzed the polarization of Lu and Lu(v13) gps in epithelial cells by confocal microscopy and domain-selective biotinylation assays. Differentiated human colon carcinoma Caco-2 cells exhibited a polarized expression of endogenous Lu antigens associated with a predominant expression of the Lu isoform at the basolateral domain of the plasma membrane and a very low expression of the Lu(v13) isoform at both the apical and basolateral domains. Analysis of transfected Madin-Darby canine kidney cells revealed a basolateral expression of Lu gp and a nonpolarized expression of Lu(v13) gp. Delivery of Lu(v13) to both apical and basolateral surfaces showed similar kinetics, indicating that this isoform is directly transported to each surface domain. A dileucine motif at position 608-609, specific to the Lu isoform, was characterized as a dominant basolateral sorting signal that prevents Lu gp from taking the apical delivery pathway.

Cell-substrate contact: another factor may influence transepithelial electrical resistance of cell layers cultured on permeable filters

Transepithelial resistance (TER) measurement has often been used to study the paracellular transport properties of epithelia grown on permeable filters, especially the barrier function of tight junctions. However, the TER value includes another source, the resistance caused by cell-substrate contact, that may give rise to a high TER value if cell-substrate separation is small. In this study we use electric cell-substrate impedance sensing (ECIS) to measure both paracellular resistance and the average cell-substrate distance of MDCK (II), HEp-2, and WI-38 VA13 cells. Comparing ECIS data with those from TER measurements of cell layers cultured on polycarbonate filters, we can obtain the approximate extra resistance resulting from cell-substrate contact for each cell type. The value of cell-substrate resistance was also estimated by two theoretical calculations that bracket the true values. Our results demonstrate that cell-substrate contact substantially influences the TER data measured using polycarbonate filters and that the extra resistance due to cell-substrate spaces depends on both cell type and filter property.

Modulation of corneal epithelial stratification by polymer surface topography

The topography and porosity of a polymer may affect the epithelialization of a corneal implant. We used an in vitro model to examine the effect of polymer surface topography on corneal epithelial tissue stratification and the deposition of proteins associated with epithelial adhesion. A range of topographies was provided by polycarbonate membranes with nominal pore diameters of 0.1, 0.4, 0.8, 1.0, 2.0, or 3.0 microm and a nonporous surface. Stratification of epithelial tissue outgrowth on these surfaces was evaluated using light and electron microscopy. Deposition of proteins associated with basement membrane and adhesion complex formation at the tissue-polymer interface was assessed using immunohistochemistry. Surfaces with pores in the 0.1-0.8-microm-diameter range supported superior stratification and protein deposition compared with those containing pores of > or = 1.0 microm. Cytoplasmic processes penetrated single pores 2.0 and 3.0 microm in diameter and fused pores 1.0 microm in diameter. Tissue on the nonporous surface had a lower level of stratification compared with surfaces with pores 0.1-0.8 microm in diameter. These results point to the significance of surface topography in biomaterial applications that require persistent epithelialization.

Tyrosine-based membrane protein sorting signals are differentially interpreted by polarized Madin-Darby canine kidney and LLC-PK1 epithelial cells

Tyrosine-dependent sequence motifs are implicated in sorting membrane proteins to the basolateral domain of Madin-Darby canine kidney (MDCK) cells. We find that these motifs are interpreted differentially in various polarized epithelial cell types. The H, K-ATPase beta subunit, which contains a tyrosine-based motif in its cytoplasmic tail, was expressed in MDCK and LLC-PK1 cells. This protein was restricted to the basolateral membrane in MDCK cells, but was localized to the apical membrane in LLC-PK1 cells. Similarly, HA-Y543, a construct in which a tyrosine-based motif was introduced into the cytoplasmic tail of influenza hemagglutinin, was sorted to the basolateral membrane of MDCK cells and retained at the apical membrane of LLC-PK1 cells. A chimera in which the cytoplasmic tail of the H,K-ATPase beta subunit protein was replaced with the analogous region of the Na,K-ATPase beta subunit polypeptide was localized to both surface domains of MDCK cells. Mutation of tyrosine-20 of the H,K-ATPase beta subunit cytoplasmic sequence to an alanine was sufficient to disrupt basolateral localization of this polypeptide. In contrast, these constructs all remain localized to the apical membrane in LLC-PK1 cells. The FcRII-B2 protein bears a di-leucine motif and is found at the basolateral membrane of both MDCK and LLC-PK1 cells. These results demonstrate that polarized epithelia are able to discriminate between different classes of specifically defined membrane protein sorting signals.

Apical polarity of N-CAM and EMMPRIN in retinal pigment epithelium resulting from suppression of basolateral signal recognition

Retinal pigment epithelial (RPE) cells apically polarize proteins that are basolateral in other epithelia. This reversal may be generated by the association of RPE with photoreceptors and the interphotoreceptor matrix, postnatal expansion of the RPE apical surface, and/or changes in RPE sorting machinery. We compared two proteins exhibiting reversed, apical polarities in RPE cells, neural cell adhesion molecule (N-CAM; 140-kD isoform) and extracellular matrix metalloproteinase inducer (EMMPRIN), with the cognate apical marker, p75-neurotrophin receptor (p75-NTR). N-CAM and p75-NTR were apically localized from birth to adulthood, contrasting with a basolateral to apical switch of EMMPRIN in developing postnatal rat RPE. Morphometric analysis demonstrated that this switch cannot be attributed to expansion of the apical surface of maturing RPE because the basolateral membrane expanded proportionally, maintaining a 3:1 apical/basolateral ratio. Kinetic analysis of polarized surface delivery in MDCK and RPE-J cells showed that EMMPRIN has a basolateral signal in its cytoplasmic tail recognized by both cell lines. In contrast, the basolateral signal of N-CAM is recognized by MDCK cells but not RPE-J cells. Deletion of N-CAM's basolateral signal did not prevent its apical localization in vivo. The data demonstrate that the apical polarity of EMMPRIN and N-CAM in mature RPE results from suppressed decoding of specific basolateral signals resulting in randomized delivery to the cell surface.

MDCK cell cultures as an epithelial in vitro model: cytoskeleton and tight junctions as indicators for the definition of age-related stages by confocal microscopy

PURPOSE

Madin Darby Canine Kidney (MDCK) cells were grown in culture, and age-related morphological changes in the cytoskeleton and tight junction (TJ) network were used to define stages in view of establishing an optimal in vitro model for the epithelial barrier.

METHODS

Growth curves and transepithelial electrical resistance (TEER) were determined, and the cytoskeleton (actin, alpha-tubulin, vimentin) and TJ (Zonula occludens proteins ZO1, ZO2) were investigated with immunofluorescent methods by confocal laser scanning microscopy (CLSM) and digital image restoration.

RESULTS

TEER measurements indicated that TJ were functional after one day. Values then remained constant. Four morphological stages could be distinguished. Stage I (0-1 day): Sub confluent cultures with flat cells; TJ established after cell-to-cell contacts are made. Stage II (2-6 days): Confluent monolayers with a complete TJ network, which remains intact throughout the later stages. Stage III (7-14 days): Rearrangement in the cytoskeleton; constant cell number; volume and surface area of cells reduced (cobble-stone appearance). Stage IV (> or = 15 days): Dome formation, i.e. thickening and spontaneous uplifting of the cell monolayer.

CONCLUSIONS

Based on the structural characteristics of stage III cell cultures, which are closest to the in vivo situation, we expect them to represent an optimal in vitro model to study drug transport and/or interactions with drugs and excipients.

Properties of a polarized primary culture from rat renal inner medullary collecting duct (IMCD) cells

A primary culture from rat renal IMCD cells was established to investigate the permeability characteristics of the luminal and contraluminal plasma membranes of the papillary collecting duct in vitro. Freshly isolated IMCD cells were grown on filters in a special "epithelial cell" medium. Confluency was proved with an epithelial volt/ohm meter. After 7 d of culture the transepithelial resistance reached more than 1000 omega x cm2. A polarization of the cells with regard to a basolateral localization of a lactate efflux system, and an L-alanine transport system was achieved. The hypotonicity-activated release systems for the organic osmolytes sorbitol and betaine were also located basolaterally, whereas taurine, glycerophosphorylcholine, and myo-inositol left the cells at both cell poles but with different capacity. Morphological observations revealed also that the monolayer was well differentiated. Thus, a model of a renal collecting duct epithelium was established which can be used to analyze polarized and differentiated transport processes across the epithelial cells and their plasma membranes.

Epithelial cell polarity affects susceptibility to Pseudomonas aeruginosa invasion and cytotoxicity

Intact tissues are relatively resistant to Pseudomonas aeruginosa-induced disease, and injury predisposes tissue to infection. Intact epithelia contain polarized cells that have distinct apical and basolateral membranes with unique lipids and proteins. In this study, the role of cell polarity in epithelial cell susceptibility to P. aeruginosa virulence mechanisms was tested. Madin-Darby canine kidney (MDCK) cells, human corneal epithelial cells, and primary cultures of two different types of airway epithelial cells were grown on Transwell filters or in plastic tissue culture wells. P. aeruginosa invasion of cells was quantified by gentamicin survival assays with two isolates that invade epithelial cells (6294 and PAO1). Cytotoxic activity was assessed by trypan blue exclusion assays with two cytotoxic strains (6206 and PA103). Basolateral surfaces of cells were exposed by one of two methods: EGTA pretreatment of epithelial cells or growth of cells in low-calcium medium. Both methods of exposing basolateral membranes increased epithelial cell susceptibility to P. aeruginosa invasion and cytotoxicity. Migrating cells were also found to be more susceptible to P. aeruginosa invasion than confluent monolayers that had established membrane polarity. Monolayers of MDCK cells that had been selected for resistance to killing by concanavalin A were resistant to both cytotoxicity and invasion by P. aeruginosa because they were more efficiently polarized for their susceptibility to P. aeruginosa virulence factors than regular MDCK cells and not because they were defective in glycosylation. These results suggest that there are factors on the basolateral surfaces of epithelial cells that promote interaction with P. aeruginosa or that there are inhibitory factors on the apical cell surface. Thus, cell polarity of intact epithelia is likely to contribute to defense against P. aeruginosa infection.

Influence of several extracellular matrix components in primary cultures of bovine mammary epithelial cells

Mammary epithelial cells, obtained from lactating cows, were cultured onto inserts coated with several components of extracellular matrix. The influence of these components upon the maintenance of differentiation has been determinated. Every day, alpha S1-casein secretion was measured by radioimmunoassay (RIA) in apical and basal compartments. Reorganization of functional tight junctions was evaluated by measurement of transepithelial electrical resistance (TER). On EHS matrix, cells underwent alveolar structures and never established TER. alpha S1-casein secretion strongly fluctuated with the day of culture. When plated onto fibronectin, cells reorganized a typical pavement and established TER. Nevertheless, TER and casein secretion highly fluctuated. On laminin-coated inserts, a few cells bound to the substratum, dedifferentiated, and proliferated to confluency within 9 days. TER progressively increased to a stable level after 15 days. Casein was not recovered after 6 days. Cells on type I collagen-coated inserts reorganized an epithelial pavement within 2 days and quickly established a stable TER. They secreted apically high levels of casein during 2 weeks. As cells maintained their biochemical differentiation, the culture on type I collagen-coated inserts seems an efficient model for primary culture of bovine mammary epithelial cells and allows studies of polarized alpha S1-casein secretion.

Differential targeting of glucosylceramide and galactosylceramide analogues after synthesis but not during transcytosis in Madin-Darby canine kidney cells

A short-chain analogue of galactosylceramide (6-NBD-amino-hexanoyl-galactosylceramide, C6-NBD-GalCer) was inserted into the apical or the basolateral surface of MDCK cells and transcytosis was monitored by depleting the opposite cell surface of the analogue with serum albumin. In MDCK I cells 32% of the analogue from the apical surface and 9% of the analogue from the basolateral surface transcytosed to the opposite surface per hour. These numbers were very similar to the flow of membrane as calculated from published data on the rate of fluid-phase transcytosis in these cells, demonstrating that C6-NBD-GalCer acted as a marker of bulk membrane flow. It was calculated that in MDCK I cells 155 microns membrane transcytosed per cell per hour in each direction. The fourfold higher percentage transported from the apical surface is explained by the apical to basolateral surface area ratio of 1:4. In MDCK II cells, with an apical to basolateral surface ratio of 1:1, transcytosis of C6-NBD-GalCer was 25% per hour in both directions. Similar numbers were obtained from measuring the fraction of endocytosed C6-NBD-GalCer that subsequently transcytosed. Under these conditions lipid leakage across the tight junction could be excluded, and the vesicular nature of lipid transcytosis was confirmed by the observation that the process was blocked at 17 degrees C. After insertion into one surface of MDCK II cells, the glucosylceramide analogue C6-NBD-GlcCer randomly equilibrated over the two surfaces in 8 h. C6-NBD-GalCer and -GlcCer transcytosed with identical kinetics. Thus no lipid selectivity in transcytosis was observed. Whereas the mechanism by which MDCK cells maintain the different lipid compositions of the two surface domains in the absence of lipid sorting along the transcytotic pathway is unclear, newly synthesized C6-NBD-GlcCer was preferentially delivered to the apical surface of MDCK II cells as compared with C6-NBD-GalCer.

Cytoplasmic juxtamembrane domain of the human EGF receptor is required for basolateral localization in MDCK cells

Although it is well established that epidermal growth factor receptors (EGFRs) are asymmetrically expressed at the basolateral plasma membrane in polarized epithelial cells, how this process is regulated is not known. The purpose of this study was to address the mechanism of directed EGFR basolateral sorting using the Madin-Darby canine kidney (MDCK) cell model. The first set of experiments established sorting patterns for endogenous canine EGFRs. The polarity of the canine EGFR was not quantitatively affected by differences in electrical resistance exhibited by the MDCK I and MDCK II cell strains. In both cases, greater than 90% of total surface EGFRs was localized to the basolateral surface. Canine EGFRs sort directly to the basolateral membrane from the trans-Golgi network with a half-time of approximately 45 min and have an approximate t1/2 of 12.5 h once reaching the basolateral surface. Human holoreceptors expressed in stably transfected MDCK cells also localize to the basolateral membrane with similar efficiency. To identify EGFR sequences necessary for basolateral sorting, MDCK cells were transfected with cDNAs coding for cytoplasmically truncated human receptor proteins. Human EGFRs truncated at Arg-651 were localized predominantly at the apical surface of filter-grown cells, whereas receptors truncated at Leu-723 were predominantly basolateral. These results suggest that the cytoplasmic juxtamembrane domain contains a positive basolateral sorting determinant. Moreover, the EGFR ectodomain or transmembrane domain may possess a cryptic sequence that specifically interacts with the apical sorting machinery once the dominant basolateral sorting signal is removed. Further elucidation of the precise location of these signals will enhance our basic understanding of regulated plasma membrane sorting, as well as the functional consequences of inappropriate EGFR expression associated with certain pathophysiologic and malignant states.

TGN38 recycles basolaterally in polarized Madin-Darby canine kidney cells

Sorting of newly synthesized plasma membrane proteins to the apical or basolateral surface domains of polarized cells is currently thought to take place within the trans-Golgi network (TGN). To explore the relationship between protein localization to the TGN and sorting to the plasma membrane in polarized epithelial cells, we have expressed constructs encoding the TGN marker, TGN38, in Madin-Darby canine kidney (MDCK) cells. We report that TGN38 is predominantly localized to the TGN of these cells and recycles via the basolateral membrane. Analyses of the distribution of Tac-TGN38 chimeric proteins in MDCK cells suggest that the cytoplasmic domain of TGN38 has information leading to both TGN localization and cycling through the basolateral surface. Mutations of the cytoplasmic domain that disrupt TGN localization also lead to nonpolarized delivery of the chimeric proteins to both surface domains. These results demonstrate an apparent equivalence of basolateral and TGN localization determinants and support an evolutionary relationship between TGN and plasma membrane sorting processes.

Expression of an endogenous asialoglycoprotein receptor in a human intestinal epithelial cell line, Caco-2

We have previously shown that rat asialoglycoprotein receptor expressed in the intestine and liver differ in mRNA size, cell surface distribution, and ratio of compositional protein subunits. In this study, we examined a well characterized intestinal epithelial cell line, Caco-2, as a potential model for studying endogenous receptor in a polarized cell line. Both subunits H1 and H2 of human asialoglycoprotein receptor were detected in Caco-2 cells by Western blots using subunit-specific antisera raised against the hepatic receptor. Antigenic receptor level in fully differentiated Caco-2 cells was approx. 1/3 to 1/2 the level of hepatic HepG2 cells H1 was the dominant subunit in both cell lines. The apparent size of H1 and H2 in Caco-2 cells was not the same as that in HepG2 cells, due to differences in N-linked glycosylation. Consistent with this finding, Northern blot analysis showed that receptor mRNA in the two cell types was of identical size. In pulse-chase experiments H1 was first detected as a 'high-mannose' precursor (40 kDa) in Caco-2 cells that was converted to mature H1 (43 kDa) with a half-life of approx. 60 min. Antigenic levels of H1 and H2 in undifferentiated Caco-2 cells were low, but increased rapidly during cell differentiation, reaching a peak level at 7 days after confluence. Immunocytochemical staining and domain-selective cell surface biotinylation assays showed that the ASGP-R was predominantly localized in the basolateral domain. The receptor in Caco-2 cells was capable of mediating specific uptake and degradation of [125I]asialoorosomucoid. The ligand uptake capacity of the basolateral surface of was approx. 10-fold higher than the apical. These characteristics (H1 subunit and basolateral predominance) of the receptor in Caco-2 cells, resembles the hepatic receptor. We conclude that Caco-2 cells endogenously express in ectopic hepatic-type functional asialoglycoprotein receptor.

Polarised interleukin 8 secretion by HT 29/19A cells

Interleukin 8 is a neutrophil chemotactic and stimulating cytokine induced by various inflammatory stimuli, including tumour necrosis factor, interleukin 1, and endotoxin. The ability of HT 29/19A enterocytes to synthesise interleukin 8 was studied. The results show that interleukin 1 is an important stimulus for interleukin 8 synthesis and secretion by HT 29/19A cells, being more potent than tumour necrosis factor. The tumour necrosis factor and interleukin 1 induced interleukin 8 secretion by HT 29/19A cells was seen to be polarised according to the direction of stimulation. These results support the concept that mucosal cells (enterocytes) may play an important part in initiating mucosal inflammation. Furthermore, it is proposed that HT 29/19A cells constitute a tool to study stimulus directed polarised cytokine secretion.