Transepithelial transport in cell culture

Fluid and solute transport by cells and a model of systemic circulation

Active fluid circulation and solute transport are essential functions of living organisms, enabling the efficient delivery of oxygen and nutrients to various physiological compartments. Since fluid circulation occurs in a network, the systemic flux and pressure are not simple outcomes of individual components. Rather, they are emergent properties of network elements and network topology. Moreover, consistent pressure and osmolarity gradients are maintained across compartments such as the kidney, interstitium, and blood vessels. The mechanisms by which these gradients and network properties are established and maintained are unanswered questions in systems physiology. Previous studies have shown that epithelial cells are fluid pumps and can actively generate pressure and osmolarity gradients. The polarization and activity of solute transporters in epithelial cells, which drive fluid flux, are influenced by pressure and osmolarity gradients. Therefore, there is an unexplored coupling between pressure and osmolarity in the circulatory network. In this work, we develop a mathematical framework that integrates the influence of pressure and osmolarity on solute transport. We use this model to explore both cellular fluid transport and systemic circulation. Using a simple network featuring the kidney-vascular interface, we show that our model naturally generates pressure and osmolarity gradients across the kidney, vessels and renal interstitium. While the current model uses this interface as an example, the findings can be generalized to other physiological compartments. This model demonstrates how systemic transport properties can depend on cellular properties and, conversely, how cell states are influenced by systemic properties. When epithelial and endothelial pumps are considered together, we predict how pressures at various points in the network depend on the overall osmolarity of the system. The model can be improved by including physiological geometries and expanding solute species, and highlights the interplay of fluid properties with cell function in living organisms.

Cell-Driven Fluid Dynamics: A Physical Model of Active Systemic Circulation

Active fluid circulation and transport are key functions of living organisms, which drive efficient delivery of oxygen and nutrients to various physiological compartments. Because fluid circulation occurs in a network, the systemic flux and pressure are not simple outcomes of any given component. Rather, they are emergent properties of network elements and network topology. Moreover, consistent pressure and osmolarity gradients across compartments such as the kidney, interstitium, and vessels are known. How these gradients and network properties are established and maintained is an unanswered question in systems physiology. Previous studies have shown that epithelial cells are fluid pumps that actively generate pressure and osmolarity gradients. Polarization and activity of ion exchangers that drive fluid flux in epithelial cells are affected by pressure and osmolarity gradients. Therefore, there is an unexplored coupling between the pressure and osmolarity in the circulating network. Here we develop a mathematical theory that integrates the influence of pressure and osmolarity on solute transport and explores both cell fluid transport and systemic circulation. This model naturally generates pressure and osmolarity gradients across physiological compartments, and demonstrates how systemic transport properties can depend on cell properties, and how the cell state can depend on systemic properties. When epithelial and endothelial pumps are considered together, we predict how pressures at various points in the network depend on the overall osmolarity of the system. The model can be improved by including physiological geometries and expanding solute species, and highlights the interplay of fluid properties with cell function in living organisms.

Bioelectric stimulation controls tissue shape and size

Epithelial tissues sheath organs and electro-mechanically regulate ion and water transport to regulate development, homeostasis, and hydrostatic organ pressure. Here, we demonstrate how external electrical stimulation allows us to control these processes in living tissues. Specifically, we electrically stimulate hollow, 3D kidneyoids and gut organoids and find that physiological-strength electrical stimulation of ∼ 5 - 10 V/cm powerfully inflates hollow tissues; a process we call electro-inflation. Electro-inflation is mediated by increased ion flux through ion channels/transporters and triggers subsequent osmotic water flow into the lumen, generating hydrostatic pressure that competes against cytoskeletal tension. Our computational studies suggest that electro-inflation is strongly driven by field-induced ion crowding on the outer surface of the tissue. Electrically stimulated tissues also break symmetry in 3D resulting from electrotaxis and affecting tissue shape. The ability of electrical cues to regulate tissue size and shape emphasizes the role and importance of the electrical micro-environment for living tissues.

Synthetic Morphogenesis: introducing IEEE journal readers to programming living mammalian cells to make structures

Synthetic morphogenesis is a new engineering discipline, in which cells are genetically engineered to make designed shapes and structures. At least in this early phase of the field, devices tend to make use of natural shape-generating processes that operate in embryonic development, but invoke them artificially at times and in orders of a technologist's choosing. This requires construction of genetic control, sequencing and feedback systems that have close parallels to electronic design, which is one reason the field may be of interest to readers of IEEE journals. The other reason is that synthetic morphogenesis allows the construction of two-way interfaces, especially opto-genetic and opto-electronic, between the living and the electronic, allowing unprecedented information flow and control between the two types of 'machine'. This review introduces synthetic morphogenesis, illustrates what has been achieved, drawing parallels wherever possible between biology and electronics, and looks forward to likely next steps and challenges to be overcome.

A novel role of PRL in regulating epithelial cell density by inducing apoptosis at confluence

Maintaining proper epithelial cell density is essential for the survival of multicellular organisms. While regulation of cell density through apoptosis is well known, its mechanistic details remain elusive. Here, we report the involvement of membrane-anchored phosphatase of regenerating liver (PRL), originally known for its role in cancer malignancy, in this process. In epithelial MDCK cells, upon confluence, doxycycline-induced expression of PRL upregulated apoptosis, reducing the cell density. This could be circumvented by artificially reducing the cell density via stretching the cell-seeded silicon chamber. Moreover, siRNA-mediated knockdown of endogenous PRL blocked apoptosis, leading to greater cell density. Mechanistically, PRL promoted apoptosis by upregulating the translation of E-cadherin and activating TGF-β pathway. Morpholino-mediated inhibition of PRL expression in zebrafish embryos caused developmental defect with reduced apoptosis and increased epithelial cell density during convergent extension. This study revealed a novel role of PRL in regulating density-dependent apoptosis in vertebrate epithelium.

Early studies on the surface epithelium of mammalian airways

This article traces the beginnings of the various areas of physiological research on airway epithelium. First mentioned in 1600, it was not until 1834 that it was found to be ciliated. Goblet and basal cells were described in 1852, to be followed by ~10 other epithelial cell types (the most recent in 2018). It also contains nerve endings and resident leukocytes. Mucociliary clearance was documented in 1835, but the first studies on the ciliary beat cycle did not appear until 1890, and a definitive description was not published until 1981. It was established in 1932 that goblet cells in the cat trachea were unresponsive to cholinergic agents; but only since 1980 or so has any significant progress been made on what does cause them to degranulate. Active transfer of salts across epithelia creates local osmotic gradients that drive transepithelial water flows. Vectorial salt transport was first described for airway epithelium in 1968, and the associated volume flows were measured in 1981. Evidence that airway epithelium releases signaling molecules first appeared in 1981. Since then, scores of molecules have been identified. The pace of research in most areas increased dramatically after the development of confluent, polarized cultures of airway epithelium in the early 1980s.

NHA2 promotes cyst development in an in vitro model of polycystic kidney disease

KEY POINTS

Significant and selective up-regulation of the Na+ /H+ exchanger NHA2 (SLC9B2) was observed in cysts of patients with autosomal dominant polycystic kidney disease. Using the MDCK cell model of cystogenesis, it was found that NHA2 increases cyst size. Silencing or pharmacological inhibition of NHA2 inhibits cyst formation in vitro. Polycystin-1 represses NHA2 expression via Ca2+ /NFAT signalling whereas the dominant negative membrane-anchored C-terminal fragment (PC1-MAT) increased NHA2 levels. Drugs (caffeine, theophylline) and hormones (vasopressin, aldosterone) known to exacerbate cysts elicit NHA2 expression. Taken together, the findings reveal NHA2 as a potential new player in salt and water homeostasis in the kidney and in the pathogenesis of polycystic kidney disease.

ABSTRACT

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 and PKD2 encoding polycystin-1 (PC1) and polycystin-2 (PC2), respectively. The molecular pathways linking polycystins to cyst development in ADPKD are still unclear. Intracystic fluid secretion via ion transporters and channels plays a crucial role in cyst expansion in ADPKD. Unexpectedly, we observed significant and selective up-regulation of NHA2, a member of the SLC9B family of Na+ /H+ exchangers, that correlated with cyst size and disease severity in ADPKD patients. Using three-dimensional cultures of MDCK cells to model cystogenesis in vitro, we showed that ectopic expression of NHA2 is causal to increased cyst size. Induction of PC1 in MDCK cells inhibited NHA2 expression with concordant inhibition of Ca2+ influx through store-dependent and -independent pathways, whereas reciprocal activation of Ca2+ influx by the dominant negative membrane-anchored C-terminal tail fragment of PC1 elevated NHA2. We showed that NHA2 is a target of Ca2+ /NFAT signalling and is transcriptionally induced by methylxanthine drugs such as caffeine and theophylline, which are contraindicated in ADPKD patients. Finally, we observed robust induction of NHA2 by vasopressin, which is physiologically consistent with increased levels of circulating vasopressin and up-regulation of vasopressin V2 receptors in ADPKD. Our findings have mechanistic implications on the emerging use of vasopressin V2 receptor antagonists such as tolvaptan as safe and effective therapy for polycystic kidney disease and reveal a potential new regulator of transepithelial salt and water transport in the kidney.

Engineering and monitoring cellular barrier models

Epithelia and endothelia delineate tissue compartments and control their environments by regulating the passage of ions and solutes. This barrier function is essential for the development and maintenance of multicellular organisms, and its dysfunction is associated with numerous human diseases. Recent advances in biomaterials and microfabrication technologies have evolved in vitro approaches for modelling biological barriers. Current microphysiological systems have become more efficient and reliable in mimicking the cell microenvironment. Additionally, methods for the quantification of barrier permeability have long provided significant insight into their underlying mechanisms. In this review, we outline the current techniques to quantify the barrier function of engineered tissues, and we also give an overview of recent microphysiological systems of biological barriers that emulate the microenvironment and microarchitecture of native tissues.

Orally Absorbed Cyclic Peptides

Peptides and proteins are not orally bioavailable in mammals, although a few peptides are intestinally absorbed in small amounts. Polypeptides are generally too large and polar to passively diffuse through lipid membranes, while most known active transport mechanisms facilitate cell uptake of only very small peptides. Systematic evaluations of peptides with molecular weights above 500 Da are needed to identify parameters that influence oral bioavailability. Here we describe 125 cyclic peptides containing four to thirty-seven amino acids that are orally absorbed by mammals. Cyclization minimizes degradation in the gut, blood, and tissues by removing cleavable N- and C-termini and by shielding components from metabolic enzymes. Cyclization also folds peptides into bioactive conformations that determine exposure of polar atoms to solvation by water and lipids and therefore can influence oral bioavailability. Key chemical properties thought to influence oral absorption and bioavailability are analyzed, including molecular weight, octanol-water partitioning, hydrogen bond donors/acceptors, rotatable bonds, and polar surface area. The cyclic peptides violated to different degrees all of the limits traditionally considered to be important for oral bioavailability of drug-like small molecules, although fewer hydrogen bond donors and reduced flexibility generally favored oral absorption.

A 3-D cell culture system to study epithelia functions using microcarriers

In vitro cell culture models used to study epithelia and epithelial diseases would benefit from the recognition that organs and tissues function in a three-dimensional (3D) environment. This context is necessary for the development of cultures that more realistically resemble in vivo tissues/organs. Our aim was to establish and characterize biologically meaningful 3D models of epithelium. We engineered 3D epithelia cultures using a kidney epithelia cell line (MDCK) and spherical polymer scaffolds. These kidney epithelia were characterized by live microscopy, immunohistochemistry and transmission electron microscopy. Strikingly, the epithelial cells displayed increased physiological relevance; they were extensively polarized and developed a more differentiated phenotype. Using such a growth system allows for direct transmission and fluorescence imaging with few restrictions using wide-field, confocal and Light Sheet Fluorescence Microscopy. We also assessed the wider relevance of this 3D culturing technique with several epithelial cell lines. Finally, we established that these 3D micro-tissues can be used for infection as well as biochemical assays and to study important cellular processes such as epithelial mesenchymal transmission. This new biomimetic model could provide a broadly applicable 3D culture system to study epithelia and epithelia related disorders.

Ketamine Increases Permeability and Alters Epithelial Phenotype of Renal Distal Tubular Cells via a GSK-3β-Dependent Mechanism

Ketamine, a dissociative anesthetic, is misused and abused worldwide as an illegal recreational drug. In addition to its neuropathic toxicity, ketamine abuse has numerous effects, including renal failure; however, the underlying mechanism is poorly understood. The process called epithelial phenotypic changes (EPCs) causes the loss of cell-cell adhesion and cell polarity in renal diseases, as well as the acquisition of migratory and invasive properties. Madin-Darby canine kidney cells, an in vitro cell model, were subjected to experimental manipulation to investigate whether ketamine could promote EPCs. Our data showed that ketamine dramatically decreased transepithelial electrical resistance and increased paracellular permeability and junction disruption, which were coupled to decreased levels of apical junctional proteins (ZO-1, occludin, and E-cadherin). Consistent with the downregulation of epithelial markers, the mesenchymal markers N-cadherin, fibronectin, and vimentin were markedly upregulated following ketamine stimulation. Of the E-cadherin repressor complexes tested, the mRNA levels of Snail, Slug, Twist, and ZEB1 were elevated. Moreover, ketamine significantly enhanced migration and invasion. Ketamine-mediated changes were at least partly caused by the inhibition of GSK-3β activity through Ser-9 phosphorylation by the PI3K/Akt pathway. Inhibiting PI3K/Akt with LY294002 reactivated GSK-3β and suppressed ketamine-enhanced permeability, EPCs, and motility. These findings were recapitulated by the inactivation of GSK-3β using the inhibitor 3F8. Taken together, these results provide evidence that ketamine induces renal distal tubular EPCs through the downregulation of several junction proteins, the upregulation of mesenchymal markers, the activation of Akt, and the inactivation of GSK-3β.

Different effects of ZO-1, ZO-2 and ZO-3 silencing on kidney collecting duct principal cell proliferation and adhesion

Coordinated cell proliferation and ability to form intercellular seals are essential features of epithelial tissue function. Tight junctions (TJs) classically act as paracellular diffusion barriers. More recently, their role in regulating epithelial cell proliferation in conjunction with scaffolding zonula occludens (ZO) proteins has come to light. The kidney collecting duct (CD) is a model of tight epithelium that displays intense proliferation during embryogenesis followed by very low cell turnover in the adult kidney. Here, we examined the influence of each ZO protein (ZO-1, -2 and -3) on CD cell proliferation. We show that all 3 ZO proteins are strongly expressed in native CD and are present at both intercellular junctions and nuclei of cultured CD principal cells (mCCDcl1). Suppression of either ZO-1 or ZO-2 resulted in increased G0/G1 retention in mCCDcl1 cells. ZO-2 suppression decreased cyclin D1 abundance while ZO-1 suppression was accompanied by increased nuclear p21 localization, the depletion of which restored cell cycle progression. Contrary to ZO-1 and ZO-2, ZO-3 expression at intercellular junctions dramatically increased with cell density and relied on the presence of ZO-1. ZO-3 depletion did not affect cell cycle progression but increased cell detachment. This latter event partly relied on increased nuclear cyclin D1 abundance and was associated with altered β1-integrin subcellular distribution and decreased occludin expression at intercellular junctions. These data reveal diverging, but interconnected, roles for each ZO protein in mCCDcl1 proliferation. While ZO-1 and ZO-2 participate in cell cycle progression, ZO-3 is an important component of cell adhesion.

Basolateral sorting of the Mg²⁺ transporter CNNM4 requires interaction with AP-1A and AP-1B

Ancient conserved domain protein/cyclin M (CNNM) 4 is an evolutionarily conserved Mg(2+) transporter that localizes at the basolateral membrane of the intestinal epithelia. Here, we show the complementary importance of clathrin adaptor protein (AP) complexes AP-1A and AP-1B in basolateral sorting of CNNM4. We first confirmed the basolateral localization of both endogenous and ectopically expressed CNNM4 in Madin-Darby Canine Kidney cells, which form highly polarized epithelia in culture. Single knockdown of μ1B, a cargo-recognition subunit of AP-1B, did not affect basolateral localization, but simultaneous knockdown of the μ1A subunit of AP-1A abrogated localization. Mutational analyses showed the importance of three conserved dileucine motifs in CNNM4 for both basolateral sorting and interaction with μ1A and μ1B. These results imply that CNNM4 is sorted to the basolateral membrane by the complementary function of AP-1A and AP-1B.

Comparison of MDCK-MDR1 and Caco-2 cell based permeability assays for anti-malarial drug screening and drug investigations

INTRODUCTION

Malaria is a major health concern and affects over 300million people a year. Accordingly, there is an urgent need for new efficacious anti-malarial drugs. A major challenge in developing new anti-malarial drugs is to design active molecules that have preferable drug-like characteristics. These "drug-like" characteristics include physiochemical properties that affect drug absorption, distribution, metabolism, and excretion (ADME). Compounds with poor ADME profiles will likely fail in vivo due to poor pharmacokinetics and/or other drug delivery related issues. There have been numerous assays developed in order to pre-screen compounds that would likely fail in further development due to poor absorption properties including PAMPA, Caco-2, and MDCK permeability assays.

METHODS

The use of cell-based permeability assays such as Caco-2 and MDCK serve as surrogate indicators of drug absorption and transport, with the two approaches often used interchangeably. We sought to evaluate both approaches in support of anti-malarial drug development. Accordingly, a comparison of both assays was conducted utilizing apparent permeability coefficient (Papp) values determined from liquid chromatography/tandem mass spectrometry (LC-MS) analyses.

RESULTS

Both Caco-2 and MDCK permeability assays produced similar Papp results for potential anti-malarial compounds with low and medium permeability. Differences were observed for compounds with high permeability and compounds that were P-gp substrates. Additionally, the utility of MDCK-MDR1 permeability measurements was demonstrated in probing the role of P-glycoprotein transport in Primaquine-Chloroquine drug-drug interactions in comparison with in vivo pharmacokinetic changes.

DISCUSSION

This study provides an in-depth comparison of the Caco-2 and MDCK-MDR1 cell based permeability assays and illustrates the utility of cell-based permeability assays in anti-malarial drug screening/development in regard to understanding transporter mediated changes in drug absorption/distribution.

Amino acids regulate transgene expression in MDCK cells

Gene expression and cell growth rely on the intracellular concentration of amino acids, which in metazoans depends on extracellular amino acid availability and transmembrane transport. To investigate the impact of extracellular amino acid concentrations on the expression of a concentrative amino acid transporter, we overexpressed the main kidney proximal tubule luminal neutral amino acid transporter B⁰AT1-collectrin (SLC6A19-TMEM27) in MDCK cell epithelia. Exogenously expressed proteins co-localized at the luminal membrane and mediated neutral amino acid uptake. However, the transgenes were lost over few cell culture passages. In contrast, the expression of a control transgene remained stable. To test whether this loss was due to inappropriately high amino acid uptake, freshly transduced MDCK cell lines were cultivated either with physiological amounts of amino acids or with the high concentration found in standard cell culture media. Expression of exogenous transporters was unaffected by physiological amino acid concentration in the media. Interestingly, mycoplasma infection resulted in a significant increase in transgene expression and correlated with the rapid metabolism of L-arginine. However, L-arginine metabolites were shown to play no role in transgene expression. In contrast, activation of the GCN2 pathway revealed by an increase in eIF2α phosphorylation may trigger transgene derepression. Taken together, high extracellular amino acid concentration provided by cell culture media appears to inhibit the constitutive expression of concentrative amino acid transporters whereas L-arginine depletion by mycoplasma induces the expression of transgenes possibly via stimulation of the GCN2 pathway.

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Transmigration route of Campylobacter jejuni across polarized intestinal epithelial cells: paracellular, transcellular or both?

Intact intercellular junctions and cellular matrix contacts are crucial structural components for the formation and maintenance of epithelial barrier functions in humans to control the commensal flora and protect against intruding microbes. Campylobacter jejuni is one of the most important zoonotic pathogens causing food-borne gastroenteritis and potentially more severe diseases such as reactive arthritis or Guillain–Barré syndrome. Crossing the intestinal epithelial barrier and host cell invasion by C. jejuni are considered to represent the primary reasons of gut tissue damage in humans and various animal model systems including monkeys, piglets, rabbits, hamsters and ferrets. C. jejuni is also able to invade underlying tissues such as the lamina propria, can enter the bloodstream, and possibly reach distinct organs such as spleen, liver or mesenteric lymph nodes. However, the molecular mechanisms as well as major bacterial and host cell factors involved in these activities are poorly understood. Various models exist by which the pathogen can trigger its own transmigration across polarized intestinal epithelial cells in vitro, the paracellular and/or transcellular mechanism. Recent studies suggest that bacterial factors such as flagellum, serine protease HtrA and lipooligosaccharide LOS may play an active role in bacterial transmigration. Here we review our knowledge on transmigration of C. jejuni as well as some other Campylobacter species, and discuss the pros and cons for the route(s) taken to travel across polarized epithelial cell monolayers. These studies provide fresh insights into the infection strategies employed by this important pathogen.

Heterogeneity of the MDCK cell line and its applicability for influenza virus research

Single-cell clones have been established from the MDCK cell line, characterized for their morphology and evaluated for their suitability for influenza virus research. Three discrete cell morphotypes were identified using light microscopy. Besides morphological features, the cell types can be distinguished by the level of expression of surface glycans recognized by peanut agglutinin (PNA). All clones were susceptible to infection by influenza viruses of different subtypes of influenza A virus (H1N1, H1N1pdm09, H3N2, H5N1) and influenza B virus, and all possessed on their surface terminally sialylated glycans with both types of glycosidic linkage (α2-3 and α2-6). The Type-1 cell lines were able to support a multicycle replication of influenza A and B viruses without help of an exogenous trypsin. In contrast, cell lines exhibiting Type-2 morphology were unable to support multicycle replication of influenza A viruses without trypsin supplementation. Western blot analysis of the hemagglutinin of H1N1 strains demonstrated that Type-2 cells were deficient in production of proteolytically activated hemagglutinin (no cleavage between HA1/HA2 was observed). HA1/HA2 cleavage of influenza B viruses in the Type-2 cells was also significantly impaired, but not completely abrogated, producing sufficient amount of activated HA to support efficient virus replication without trypsin. In contrast, all clones of Type-1 cells were able to produce proteolytically activated hemagglutinin of influenza A and B viruses. However, the growth kinetics and plaque size of influenza A viruses varied significantly in different clones. Influenza B virus also showed different plaque size, with the biggest plaque formation in the Type-2 cells, although the growth kinetics and peak infectivity titers were similar in all clones. Taken together, the study demonstrates that the population of original MDCK cells is represented by various types of cells that differ in their capacities to support replication of influenza A and B viruses.

Monitoring of the dynamics of epithelial dome formation using a novel culture chamber for long-term continuous live-cell imaging

Epithelial tissue guarantees proper performance of many organs, e.g., the kidneys, the gastrointestinal organs, and endocrine glands. Epithelial layers are responsible for the formation and maintenance of separate compartments with distinct solute composition. This is achieved by epithelial layers forming a barrier between the two compartments and concomitantly allowing site-directed transepithelial transport, uptake or secretion of electrolytes, energy substrates, proteins, and other solutes.Research on epithelial tissue functions has highly profited from the establishment of tissue culture technologies allowing to cultivate primary epithelial cells or established epithelial cell lines. A property of transporting epithelia cultured in vitro that has long been noted is the formation of the so-called domes on solid growth supports, which represent fluid filled blisters between the solid growth surface and the cell layer. Formation of domes is regarded as a sign of active transport processes and an intact epithelial barrier function due to functional tight junctional cell-cell contacts. A novel methodology for long-term live-cell light microscopy is described in the present article, which allows the monitoring of the dynamic nature of structures, such as epithelial domes over days to weeks of tissue culture ("under the microscope").

Could fresh human tissues play a key role in drug development?

Biopta was founded in 2002, to provide human tissue-based drug development and testing services to the pharmaceutical industry. Although animal tissues are readily available and are relatively inexpensive, they frequently fail to faithfully predict the results seen in the clinic. Human tissues can provide integrated responses to test drugs in a manner more representative than individual cell types or cell lines alone, and more-directly relevant to the species of interest -- Homo sapiens. In order to expand the use of human tissues, however, an improved infrastructure for the collection and distribution of fresh, functional tissues is highly desirable. Moreover, where there is the potential to obtain tissue from various locations, it becomes possible to test tissue that is specific to the site of drug activity. This is important, as differences may occur between the same tissue types in different locations in the body. The detection of adverse effects is greatly helped by knowledge of how existing drugs behave in the human body. These drugs can act as reference compounds, so that new compounds can then be compared, by using standard concentration-response type studies, in a huge variety of tissues, and their effects extrapolated from what is known of the reference compounds.

Beyond structure, to survival: activation of Stat3 by cadherin engagement

Cells in normal tissues or in tumors have extensive opportunities for adhesion to their neighbors and the importance of cell to cell contact in the study of fundamental cellular processes is beginning to emerge. In this review, we discuss recent evidence of dramatic changes in the activity of an important signal transducer found to be profoundly affected by cell to cell adhesion, the signal transducer and activator of transcription-3 (Stat3). Direct cadherin engagement, growth of cells to postconfluence, or formation of multicellular aggregates were found to induce a striking increase in the levels of Stat3 activity, Rac1/Cdc42, and members of the IL6 receptor family in different settings. This activation was specific to Stat3, in that the levels of the extracellular signal regulated kinase (Erk1/2), a signal transducer often coordinately activated with Stat3 by a number of growth factors or oncogenes, remained unaffected by cell density. Density-dependent Stat3 activation may play a key role in survival, and could contribute to the establishment of cell polarity. It is clear that at any given time the total Stat3 activity levels in a cell are the sum of the effects of cell to cell adhesion plus the conventional Stat3 activating factors present.

In vitro solubility, stability and permeability of novel quercetin-amino acid conjugates

In order to discover a quercetin prodrug with improved bioavailability, we synthesized nine quercetin-amion acid conjugates and estimated their pharmacokinetic properties including water solubility, stability against chemical or enzymatic hydrolysis, and cell permeability. Among the synthesized quercetin prodrugs, quercetin-glutamic acid conjugate Qu-E (4g/5g) showed remarkable increases in water solubility, stability, and cell permeability compared with quercetin, which warrants further development as a quercetin prodrug.

Quantum dots and nanoparticles for photodynamic and radiation therapies of cancer

Semiconductor quantum dots and nanoparticles composed of metals, lipids or polymers have emerged with promising applications for early detection and therapy of cancer. Quantum dots with unique optical properties are commonly composed of cadmium contained semiconductors. Cadmium is potentially hazardous, and toxicity of such quantum dots to living cells, and humans, is not yet systematically investigated. Therefore, search for less toxic materials with similar targeting and optical properties is of further interest. Whereas, the investigation of luminescence nanoparticles as light sources for cancer therapy is very interesting. Despite advances in neurosurgery and radiotherapy the prognosis for patients with malignant gliomas has changed little for the last decades. Cancer treatment requires high accuracy in delivering ionizing radiation to reduce toxicity to surrounding tissues. Recently some research has been focused in developing photosensitizing quantum dots for production of radicals upon absorption of visible light. In spite of the fact that visible light is safe, this approach is suitable to treat only superficial tumours. Ionizing radiation (X-rays and gamma rays) penetrate much deeper thus offering a big advantage in treating patients with tumours in internal organs. Such concept of using quantum dots and nanoparticles to yield electrons and radicals in photodynamic and radiation therapies as well their combination is reviewed in this article.

Cell confluency-induced Stat3 activation regulates NHE3 expression by recruiting Sp1 and Sp3 to the proximal NHE3 promoter region during epithelial dome formation

Activation of signal transducer and activator of transcription-3 (Stat3) during cell confluency is related to its regulatory roles in cell growth arrest- or survival-related physiological or developmental processes. We previously demonstrated that this signaling event triggers epithelial dome formation by transcriptional augmentation of sodium hydrogen exchanger-3 (NHE3) expression. However, the detailed molecular mechanism remained unclear. By using serial deletions, site-directed mutagenesis, and EMSA analysis, we now demonstrate Stat3 binding to an atypical Stat3-response element in the rat proximal NHE3 promoter, located adjacent to a cluster of Sp cis-elements (SpA/B/C), within -77/-36 nt of the gene. SpB (-58/-55 nt) site was more effective than SpA (-72/-69 nt) site for cooperative binding of Sp1/Sp3. Increasing cell density had no effect on Sp1/Sp3 expression but resulted in their increased binding to the SpA/B/C probe along with Stat3 and concurrently with enhanced nuclear pTyr705-Stat3 level. Immunoprecipitation performed with the nuclear extracts demonstrated physical interaction of Stat3 and Sp1/Sp3 triggered by cell confluency. Stat3 inhibition by overexpression of dominant-negative Stat3-D mutant in MDCK cells or by small interfering RNA-mediated knockdown in Caco-2 cells resulted in inhibition of the cell density-induced NHE3 expression, Sp1/Sp3 binding, and NHE3 promoter activity and in decreased dome formation. Thus, during confluency, ligand-independent Stat3 activation leads to its interaction with Sp1/Sp3, their recruitment to the SpA/B/C cluster in a Stat3 DNA-binding domain-dependent fashion, increased transcription, and expression of NHE3, to coordinate cell density-mediated epithelial dome formation.

Biogenesis of epithelial cell plasma membranes

Polarized monolayers of cultured epithelial cells, such as the kidney-derived MDCK cell line, when infected with enveloped viruses, provide a convenient model system for study of the intracellular routes followed by newly synthesized glycoproteins to reach specific domains of the plasma membrane. The polarized nature of the monolayers is reflected in the asymmetric assembly of enveloped viruses, some of which, such as influenza and simian virus 5 (SV5), bud from the apical surfaces of the cells, while others, such as vesicular stomatitis virus (VSV), emerge from the basolateral surfaces. MDCK cells can sustain double infection with viruses of different budding polarity, and within such cells the envelope glycoproteins of the two viruses are synthesized simultaneously and assembled into virions at different sites. Immunoelectron microscopic observations of doubly infected cells show that glycoproteins of influenza and VSV traverse the same Golgi apparatus. This indicates that critical sorting steps must take place during or after passage of the glycoproteins through the organelle. Following passage through the Golgi, the HA glycoprotein accumulates almost exclusively at the apical surface, where the influenza virions assemble. Significant amounts of the G protein, however, are detected on both plasma membranes in singly and doubly infected cells, although VSV virion assembly is limited to basolateral domains. These observations indicate that the site of VSV budding is not exclusively determined by the presence of G polypeptides on a given cell-surface domain. It is possible that other cellular or viral components are responsible for the selection of the appropriate budding domain or that the G protein found on the apical surface must be transferred to the basolateral domain before it becomes competent for assembly.

Chapter 6 Protein Sorting in the Secretory Pathway

This chapter focuses on protein sorting in the secretory pathway. From primary and secondary biosynthetic sites in the cytosol and mitochondrial matrix, respectively, proteins and lipids are distributed to more than 30 final destinations in membranes or membrane-bound spaces, where they carry out their programmed function. Molecular sorting is defined, in its most general sense, as the sum of the mechanisms that determine the distribution of a given molecule from its site of synthesis to its site of function in the cell. The final site of residence of a protein in a eukaryotic cell is determined by a combination of various factors, acting in concert: (1) site of synthesis, (2) sorting signals or zip codes, (3) signal recognition or decoding mechanisms, (4) cotranslational or posttranslational mechanisms for translocation across membranes, (5) specific fusion-fission interactions between intracellular vesicular compartments, and (6) restrictions to the lateral mobility in the plane of the bilayer. Improvements in cell fractionation, protein separation, and immune precipitation procedures in the past decade have made them possible. Very little is known about the mechanisms that mediate the localization and concentration of specific proteins and lipids within organelles. Various experimental model systems have become available for their study. The advent of recombinant DNA technology has shortened the time needed for obtaining the primary structure of proteins to a few months.

Depletion of apical transport proteins perturbs epithelial cyst formation and ciliogenesis

Epithelial cells are vital for maintaining the complex architecture and functions of organs in the body. Directed by cues from the extracellular matrix, cells polarize their surface into apical and basolateral domains, and connect by extensive cell-cell junctions to form tightly vowen epithelial layers. In fully polarized cells, primary cilia project from the apical surface. Madin-Darby canine kidney (MDCK) cells provide a model to study organization of cells as monolayers and also in 3D in cysts. In this study retrovirus-mediated RNA interference (RNAi) was used to generate a series of knockdowns (KDs) for proteins implicated in apical transport: annexin-13, caveolin-1, galectin-3, syntaxin-3, syntaxin-2 and VIP17 and/or MAL. Cyst cultures were then employed to study the effects of these KDs on epithelial morphogenesis. Depletion of these proteins by RNAi stalled the development of the apical lumen in cysts and resulted in impaired ciliogenesis. The most severe ciliary defects were observed in annexin-13 and syntaxin-3 KD cysts. Although the phenotypes demonstrate the robustness of the formation of the polarized membrane domains, they indicate the important role of apical membrane biogenesis in epithelial organization.

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.

Characterization of a long-term rat mTAL cell line

A medullary thick ascending limb (mTAL) cell line, termed raTAL, has been established from freshly isolated rat mTAL tubules and cultured continuously for up to 75 passages; it retains characteristics of mTAL cells even after retrieval from storage in liquid nitrogen for several months. The cells express Tamm-Horsfall glycoprotein (THP), a TAL-specific marker, grow to confluence, exhibit a polygonal morphology characteristic of epithelial cells, and form "domes." Detection of THP, Na(+)-K(+)-2Cl(-) cotransporter (NKCC2), Na(+)-K(+)-ATPase, and renal outer medullary K(+) channel (ROMK) was achieved using indirect immunofluorescence and confocal microscopy. Western blot analysis of NKCC2 expression using two different antibodies revealed a band of approximately 160 kDa, and RT-PCR analysis demonstrated the presence of NKCC2 isoforms A and F, which was confirmed by DNA sequencing; transport of Cl(-) into raTAL cells was inhibited by furosemide. Ouabain- and bumetanide-sensitive oxygen consumption, an index of ion transport activity in the mTAL, was observed in raTAL cells, and the number of domes present was reduced significantly when cells were incubated in the presence of ouabain or bumetanide. The specific activity of Na(+)-K(+)-ATPase activity was determined in raTAL cells (0.67 +/- 0.18 nmol P(i).microg protein(-1).min(-1)), primary cultures of mTAL cells (0.39 +/- 0.08 nmol P(i).microg protein(-1).min(-1)), and freshly isolated mTAL tubules (1.10 +/- 0.29 nmol P(i).microg protein(-1).min(-1)), and approximately 30-50% of total cellular ATPase activity was inhibited by ouabain, in accord with other mTAL preparations. This cell line will be used in studies that address biochemical, molecular, and physiological mechanisms in the mTAL.

Four-dimensional imaging of filter-grown polarized epithelial cells

Understanding how epithelial cells generate and maintain polarity and function requires live cell imaging. In order for cells to become fully polarized, it is necessary to grow them on a permeable membrane filter; however, the translucent filter obstructs the microscope light path required for quantitative live cell imaging. Alternatively, the membrane filter may be excised but this eliminates selective access to apical and basolateral surfaces. Conversely, epithelial cells cultured directly on glass exhibit different phenotypes and functions from filter grown cells. Here, we describe a new method for culturing polarized epithelial cells on a Transwell filter insert that allows superior live cell imaging with spatial and temporal image resolution previously unachievable using conventional methods. Cells were cultured on the underside of a filter support. Epithelial cells grown in this inverted configuration exhibit a fully polarized architecture, including the presence of functional tight junctions. This new culturing system permits four-dimensional (three spatial dimension over time) imaging of endosome and Golgi apparatus dynamics, and permits selective manipulation of the apical and basolateral surfaces. This new technique has wide applicability for visualization and manipulation of polarized epithelial cells.

Cell confluence-induced activation of signal transducer and activator of transcription-3 (Stat3) triggers epithelial dome formation via augmentation of sodium hydrogen exchanger-3 (NHE3) expression

Cell confluence induces the activation of signal transducer and activator of transcription-3 (Stat3) in various cancer and epithelial cells, yet the biological implications and the associated regulatory mechanisms remain unclear. Because confluent polarized epithelia demonstrate dome formation and sodium influx that mimic the onset of differentiation, we sought to elucidate the role of Stat3 in association with the regulation of selective epithelial transporters in this biological phenomenon. This study established the correlation between Stat3 activation and cell confluence-induced dome formation in Madin-Darby canine kidney cells (MDCK) by following Stat3 activation events in dome-forming cells. Epifluorescent and confocal microscopy provided evidence showing specific localization of phosphorylated Stat3 Tyr(705) in the nuclei of dome-forming cells at initial stages. The relationship was further elucidated by the establishment of tetracycline-inducible expression of constitutive Stat3 mutant (Stat3-C) in MDCK cells or expression of dominant negative Stat3 (Stat3-D) stable cell lines (MDCK and NMuMG). Dome formation was promoted by the expression of Stat3-C but inhibited by Stat3-D. Two trans-epithelial transporters, NHE3 and ENaC alpha-subunit, were found to be increased during cell confluence. Interestingly, NHE3 expression could be specifically up-regulated by Stat3-C but inhibited by Stat3-D through promoter regulation, whereas NHE1 and ENaC alpha-subunit were not affected by Stat3 expression. Application of NHE3 shRNA, NHE3 inhibitors (EIPA and S3226) suppressed confluence-induced dome formation in MDCK or NMuMG cells. These results demonstrate a cell confluence-induced Stat3 signaling pathway in epithelial cells in triggering dome formation through NHE3 augmentation.

Renal nucleoside transporters: physiological and clinical implicationsThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Membrane Proteins in Health and Disease

Renal handling of physiological and pharmacological nucleosides is a major determinant of their plasma levels and tissue availabilities. Additionally, the pharmacokinetics and normal tissue toxicities of nucleoside drugs are influenced by their handling in the kidney. Renal reabsorption or secretion of nucleosides is selective and dependent on integral membrane proteins, termed nucleoside transporters (NTs) present in renal epithelia. The 7 known human NTs (hNTs) exhibit varying permeant selectivities and are divided into 2 protein families: the solute carrier (SLC) 29 (SLC29A1, SLC29A2, SLC29A3, SLC29A4) and SLC28 (SLC28A1, SLC28A2, SLC28A3) proteins, otherwise known, respectively, as the human equilibrative NTs (hENTs, hENT1, hENT2, hENT3, hENT4) and human concentrative NTs (hCNTs, hCNT1, hCNT2, hCNT3). The well characterized hENTs (hENT1 and hENT2) are bidirectional facilitative diffusion transporters in plasma membranes; hENT3 and hENT4 are much less well known, although hENT3, found in lysosomal membranes, transports nucleosides and is pH dependent, whereas hENT4–PMAT is a H+-adenosine cotransporter as well as a monoamine–organic cation transporter. The 3 hCNTs are unidirectional secondary active Na+-nucleoside cotransporters. In renal epithelial cells, hCNT1, hCNT2, and hCNT3 at apical membranes, and hENT1 and hENT2 at basolateral membranes, apparently work in concert to mediate reabsorption of nucleosides from lumen to blood, driven by Na+ gradients. Secretion of some physiological nucleosides, therapeutic nucleoside analog drugs, and nucleotide metabolites of therapeutic nucleoside and nucleobase drugs likely occurs through various xenobiotic transporters in renal epithelia, including organic cation transporters, organic anion transporters, multidrug resistance related proteins, and multidrug resistance proteins. Mounting evidence suggests that hENT1 may have a presence at both apical and basolateral membranes of renal epithelia, and thus may participate in both selective secretory and reabsorptive fluxes of nucleosides. In this review, the renal handling of nucleosides is examined with respect to physiological and clinical implications for the regulation of human kidney NTs and adenosine signaling, intracellular nucleoside transport, and nephrotoxicities associated with some nucleoside drugs.

Establishment and characterization of a novel polarized MDCK epithelial cellular model for CFTR studies

F508del is the most common mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that is responsible for the genetic disease Cystic Fibrosis (CF). It results in a major failure of CFTR to traffic to the apical membrane of epithelial cells, where it should function as a chloride (Cl-) channel. Most studies on localization, processing and cellular trafficking of wild-type (wt) and F508del-CFTR have been performed in non-epithelial cells. Notwithstanding, polarized epithelial cells possess distinctly organized and regulated membrane trafficking pathways. We have used Madin-Darby canine kidney (MDCK) type II cells (proximal tubular cells which do not express endogenous CFTR) to generate novel epithelial, polarized cellular models stably expressing wt- or F508del-CFTR through transduction with recombinant lentiviral vectors. Characterization of these cell lines shows that wt-CFTR is correctly processed and apically localized, producing a cAMP-activated Cl- conductance. In contrast, F508del-CFTR is mostly detected in itsimmature form, localized intracellularly and producing only residual Cl- conductance. These novel cell lines constitute bona fide models and significantly improved resources to investigate the molecular mechanisms of polarized membrane traffic of wt- and F508del-CFTR in the same cellular background. They are also useful to identify/validate novel therapeutic compounds for CF.

IN AWE OF SUBCELLULAR COMPLEXITY: 50 Years of Trespassing Boundaries Within the Cell

In this review I describe the several stages of my research career, all of which were driven by a desire to understand the basic mechanisms responsible for the complex and beautiful organization of the eukaryotic cell. I was originally trained as an electron microscopist in Argentina, and my first major contribution was the introduction of glutaraldehyde as a fixative that preserved the fine structure of cells, which opened the way for cytochemical studies at the EM level. My subsequent work on membrane-bound ribosomes illuminated the process of cotranslational translocation of polypeptides across the ER membrane and led to the formulation, with Gunter Blobel, of the signal hypothesis. My later studies with many talented colleagues contributed to an understanding of ER structure and function and aspects of the mechanisms that generate and maintain the polarity of epithelial cells. For this work my laboratory introduced the now widely adopted Madin-Darby canine kidney (MDCK) cell line, and demonstrated the polarized budding of envelope viruses from those cells, providing a powerful new system that further advanced the field of protein traffic.

Increased association with detergent-resistant membranes/lipid rafts of apically targeted mutants of the interleukin-6 receptor gp80

Interleukin (IL)-6 is an important cytokine in inflammatory processes, differentiation and growth. The IL-6 receptor complex comprises the specific IL-6 receptor (gp80) and two molecules of the signal tranducing component gp130 which transduces the signal into the nucleus via the Jak-STAT pathway. Both, gp80 and gp130 are sorted preferentially to the basolateral membrane of polarised Madin-Darby canine kidney (MDCK) cells. Previously, we have shown that gp130 partially localises to detergent-resistant membranes (DRMs)/lipid rafts and that lipid raft integrity is crucial for signalling to occur. Here we now demonstrate that wild-type gp80 is associated with DRMs only to a minor extent. However, gp80 mutants which lack parts of the cytoplasmic domain and therefore are more apically expressed than the wild type show an increased affinity for the liquid-ordered membrane domain. Studies with non-polarised MDCK cells suggest that the lipid raft association of the different mutants of gp80 precedes the establishment of cell polarity. Our findings suggest that lipid rafts play a role in the sorting of apically targeted gp80.

Effect of desipramine on Ca2+ levels and growth in renal tubular cells

The in vitro effect of desipramine on renal tubular cell is unknown. In Madin-Darby canine kidney (MDCK) cells, the effect of desipramine on intracellular Ca2+ concentration ([Ca2+]i) was measured by using fura-2. Desipramine (>25 microM) caused a rapid and sustained rise of [Ca2+]i in a concentration-dependent manner (EC50=50 microM). Desipramine-induced [Ca2+]i rise was prevented by 40% by removal of extracellular Ca2+ but was not altered by L-type Ca2+ channel blockers. In Ca2+-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, caused a monophasic [Ca2+]i rise, after which desipramine failed to release more Ca2+; in addition, pretreatment with desipramine partly decreased thapsigargin-induced [Ca2+]i increase. U73122, an inhibitor of phospholipase C, did not change desipramine-induced [Ca2+]i rise. Incubation with 10-100 microM desipramine enhances or inhibits cell proliferation in a concentration- and time-dependent manner. The inhibitory effect of desipramine on proliferation was not extracellular Ca2+-dependent. Apoptosis appears to contribute to desipramine-induced cell death. Together, these findings suggest that desipramine increases baseline [Ca2+]i in renal tubular cells by evoking both extracellular Ca2+ influx and intracellular Ca2+ release, and can cause apoptosis.

Organization of vesicular trafficking in epithelia

Experiments using mammalian epithelial cell lines have elucidated biosynthetic and recycling pathways for apical and basolateral plasma-membrane proteins, and have identified components that guide apical and basolateral proteins along these pathways. These components include apical and basolateral sorting signals, adaptors for basolateral signals, and docking and fusion proteins for vesicular trafficking. Recent live-cell-imaging studies provide a real-time view of sorting processes in epithelial cells, including key roles for actin, microtubules and motors in the organization of post-Golgi trafficking.

Metabolic cleavage of cell-penetrating peptides in contact with epithelial models: human calcitonin (hCT)-derived peptides, Tat(47-57) and penetratin(43-58)

We assessed the metabolic degradation kinetics and cleavage patterns of some selected CPP (cell-penetrating peptides) after incubation with confluent epithelial models. Synthesis of N-terminal CF [5(6)-carboxyfluorescein]-labelled CPP, namely hCT (human calcitonin)-derived sequences, Tat(47-57) and penetratin(43-58), was through Fmoc (fluoren-9-ylmethoxycarbonyl) chemistry. Metabolic degradation kinetics of the tested CPP in contact with three cell-cultured epithelial models, MDCK (Madin-Darby canine kidney), Calu-3 and TR146, was evaluated by reversed-phase HPLC. Identification of the resulting metabolites of CF-hCT(9-32) was through reversed-phase HPLC fractionation and peak allocation by MALDI-TOF-MS (matrix-assisted laser-desorption ionization-time-of-flight mass spectrometry) or direct MALDI-TOF-MS of incubates. Levels of proteolytic activity varied highly between the investigated epithelial models and the CPP. The Calu-3 model exhibited the highest proteolytic activity. The patterns of metabolic cleavage of hCT(9-32) were similar in all three models. Initial cleavage of this peptide occurred at the N-terminal domain, possibly by endopeptidase activity yielding both the N- and the C-terminal counterparts. Further metabolic degradation was by aminopeptidase, endopeptidase and/or carboxypeptidase activities. In conclusion, when in contact with epithelial models, the studied CPP were subject to efficient metabolism, a prerequisite of cargo release on the one hand, but with potential for premature cleavage and loss of the cargo as well on the other. The results, particularly on hCT(9-32), may be used as a template to suggest structural modifications towards improved CPP performance.

Effects of Saponins from the Root Bark of Aralia elata on the Transport of Chondroitin Sulfate in Caco-2 Cell Monolayers and Rats

We investigated the intestinal absorption enhancing effect of the saponins from the root bark of Aralia elata (SRBAE) in Caco-2 cell monolayers and rats. SRBAE at concentrations of 0.04% and 0.08% (w/v) decreased the transepithelial electrical resistance (TEER) values and increased the paracellular uptake of chondroitin sulfates (CSs) having different molecular weights (MW 500, 4500, and 18000) in a dose-dependent manner. We also evaluated the cytotoxicity of SRBAE to determine its proper concentration as an absorption enhancer. MTT assay and trypan blue exclusion test indicated that the cytotoxicity of SRBAE at concentrations of 0.04% and 0.08% was negligible. CS (MW 18000) was orally administered with or without SRBAE to rats. The oral administration of SRBAE (250 mg/kg) in 1 h increased the intestinal absorption of CS, by 4.9-fold versus the control (CS alone). Histological examination of the gastrointestinal tissues showed that SRBAE did not cause any damage to tissues. In conclusion, our results suggest that SRBAE acts as an efficient absorption enhancer and makes it easier for hydrophilic molecules to penetrate the intestinal epithelium.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

Vitamin D3upregulates plasma membrane Ca2+-ATPase expression and potentiates apico-basal Ca2+flux in MDCK cells

Plasma membrane Ca2+-ATPases (PMCAs) are a ubiquitous system for the expulsion of Ca2+from eukaryotic cells. In tight monolayers of polarized Madin-Darby canine kidney (MDCK) cells representing a distal kidney tubule model, PMCAs are responsible for about one-third of the vectorial Ca2+transport under resting conditions, with the remainder being provided by the Na+/Ca2+exchanger. Vitamin D3(VitD) is known to increase PMCA expression and activity in Ca2+-transporting tissues such as the intestine, as well as in osteoblasts and Madin-Darby bovine kidney epithelial cells. We found that VitD upregulated the expression of the PMCAs (mainly PMCA4b) in MDCK cell lysates at the RNA and protein level in a time- and dose-dependent manner. Interestingly, VitD caused a decrease of the PMCAs in the apical plasma membrane fraction and a concomitant increase of the pumps in the basolateral membrane. Functional studies demonstrated that transcellular45Ca2+flux from the apical-to-basolateral compartment was significantly enhanced by VitD. These findings demonstrate that VitD is a positive regulator of the PMCAs in MDCK epithelial cells. The correlation of decreased apical/increased basolateral expression of the PMCAs with an increase in transcellular Ca2+flux from the apical (urine) toward the basolateral (blood) compartment indicates the physiological relevance of VitD function in kidney tubular Ca2+reabsorption.