Tube morphogenesis: making and shaping biological tubes

From cells to organs: building polarized tissue

How do animal cells assemble into tissues and organs? A diverse array of tissue structures and shapes can be formed by organizing groups of cells into different polarized arrangements and by coordinating their polarity in space and time. Conserved design principles underlying this diversity are emerging from studies of model organisms and tissues. We discuss how conserved polarity complexes, signalling networks, transcription factors, membrane-trafficking pathways, mechanisms for forming lumens in tubes and other hollow structures, and transitions between different types of polarity, such as between epithelial and mesenchymal cells, are used in similar and iterative manners to build all tissues.

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.

Molecular architecture of the fruit fly's airway epithelial immune system

BACKGROUND

Airway epithelial cells not only constitute a physical barrier, but also the first line of defence against airborne pathogens. At the same time, they are constantly exposed to reactive oxygen species. Therefore, airway epithelia cells have to possess a sophisticated innate immune system and a molecular armamentarium to detoxify reactive oxygen species. It has become apparent that deregulation of epithelial innate immunity is a major reason for the development of chronic inflammatory lung diseases. To elucidate the molecular architecture of the innate immune system of airway epithelial cells, we choose the fruit fly Drosophila melanogaster as a model, because it has the simplest type of airways, consisting of epithelial cells only. Elucidating the structure of the innate immune system of this "airway epithelial cell culture" might enable us to understand why deregulatory processes in innate immune signalling cascades lead to long lasting inflammatory events.

RESULTS

All airway epithelial cells of the fruit fly are able to launch an immune response. They contain only one functional signal transduction pathway that converges onto NF-kappaB factors, namely the IMD-pathway, which is homologous to the TNF-alpha receptor pathway. Although vital parts of the Toll-pathway are missing, dorsal and dif, the NF-kappaB factors dedicated to this signalling system, are present. Other pathways involved in immune regulation, such as the JNK- and the JAK/STAT-pathway, are completely functional in these cells. In addition, most peptidoglycan recognition proteins, representing the almost complete collection of pattern recognition receptors, are part of the epithelial cells equipment. Potential effector molecules are different antimicrobial peptides and lysozymes, but also transferrin that can inhibit bacterial growth through iron-depletion. Reactive oxygen species can be inactivated through the almost complete armamentarium of enzymatic antioxidants that has the fly to its disposal.

CONCLUSION

The innate immune system of the fly's airway epithelium has a very peculiar organization. A great variety of pattern recognition receptors as well as of potential effector molecules are conspicuous, whereas signalling presumably occurs through a single NF-kappaB activating pathway. This architecture will allow reacting if confronted with different bacterial or fungal elicitors by activation of a multitude of effectors.

gammaCOP is required for apical protein secretion and epithelial morphogenesis in Drosophila melanogaster

Background

There is increasing evidence that tissue-specific modifications of basic cellular functions play an important role in development and disease. To identify the functions of COPI coatomer-mediated membrane trafficking in Drosophila development, we were aiming to create loss-of-function mutations in the γCOP gene, which encodes a subunit of the COPI coatomer complex.

Principal Findings

We found that γCOP is essential for the viability of the Drosophila embryo. In the absence of zygotic γCOP activity, embryos die late in embryogenesis and display pronounced defects in morphogenesis of the embryonic epidermis and of tracheal tubes. The coordinated cell rearrangements and cell shape changes during tracheal tube morphogenesis critically depend on apical secretion of certain proteins. Investigation of tracheal morphogenesis in γCOP loss-of-function mutants revealed that several key proteins required for tracheal morphogenesis are not properly secreted into the apical lumen. As a consequence, γCOP mutants show defects in cell rearrangements during branch elongation, in tube dilation, as well as in tube fusion. We present genetic evidence that a specific subset of the tracheal defects in γCOP mutants is due to the reduced secretion of the Zona Pellucida protein Piopio. Thus, we identified a critical target protein of COPI-dependent secretion in epithelial tube morphogenesis.

Conclusions/Significance

These studies highlight the role of COPI coatomer-mediated vesicle trafficking in both general and tissue-specific secretion in a multicellular organism. Although COPI coatomer is generally required for protein secretion, we show that the phenotypic effect of γCOP mutations is surprisingly specific. Importantly, we attribute a distinct aspect of the γCOP phenotype to the effect on a specific key target protein.

Biochemical and structural analysis of alpha-catenin in cell-cell contacts

Cadherins are transmembrane adhesion molecules that mediate homotypic cell-cell contact. In adherens junctions, the cytoplasmic domain of cadherins is functionally linked to the actin cytoskeleton through a series of proteins known as catenins. E-cadherin binds to beta-catenin, which in turn binds to alpha-catenin to form a ternary complex. alpha-Catenin also binds to actin, and it was assumed previously that alpha-catenin links the cadherin-catenin complex to actin. However, biochemical, structural and live-cell imaging studies of the cadherin-catenin complex and its interaction with actin show that binding of beta-catenin to alpha-catenin prevents the latter from binding to actin. Biochemical and structural data indicate that alpha-catenin acts as an allosteric protein whose conformation and activity changes depending on whether or not it is bound to beta-catenin. Initial contacts between cells occur on dynamic lamellipodia formed by polymerization of branched actin networks, a process controlled by the Arp2/3 (actin-related protein 2/3) complex. alpha-Catenin can suppress the activity of Arp2/3 by competing for actin filaments. These findings lead to a model for adherens junction formation in which clustering of the cadherin-beta-catenin complex recruits high levels of alpha-catenin that can suppress the Arp2/3 complex, leading to cessation of lamellipodial movement and formation of a stable contact. Thus alpha-catenin appears to play a central role in cell-cell contact formation.

The first "Slit" is the deepest: the secret to a hollow heart

Tubular organs are essential for life, but lumen formation in nonepithelial tissues such as the vascular system or heart is poorly understood. Two studies in this issue (Medioni, C., M. Astier, M. Zmojdzian, K. Jagla, and M. Sémériva. 2008. J. Cell Biol. 182:249-261; Santiago-Martínez, E., N.H. Soplop, R. Patel, and S.G. Kramer. 2008. J. Cell Biol. 182:241-248) reveal unexpected roles for the Slit-Robo signaling system during Drosophila melanogaster heart morphogenesis. In cardioblasts, Slit and Robo modulate the cell shape changes and domains of E-cadherin-based adhesion that drive lumen formation. Furthermore, in contrast to the well-known paracrine role of Slit and Robo in guiding cell migrations, here Slit and Robo may act by autocrine signaling. In addition, the two groups demonstrate that heart lumen formation is even more distinct from typical epithelial tubulogenesis mechanisms because the heart lumen is bounded by membrane surfaces that have basal rather than apical attributes. As the D. melanogaster cardioblasts are thought to have significant evolutionary similarity to vertebrate endothelial and cardiac lineages, these findings are likely to provide insights into mechanisms of vertebrate heart and vascular morphogenesis.

Genetic control of cell morphogenesis during Drosophila melanogaster cardiac tube formation

Tubulogenesis is an essential component of organ development, yet the underlying cellular mechanisms are poorly understood. We analyze here the formation of the Drosophila melanogaster cardiac lumen that arises from the migration and subsequent coalescence of bilateral rows of cardioblasts. Our study of cell behavior using three-dimensional and time-lapse imaging and the distribution of cell polarity markers reveals a new mechanism of tubulogenesis in which repulsion of prepatterned luminal domains with basal membrane properties and cell shape remodeling constitute the main driving forces. Furthermore, we identify a genetic pathway in which roundabout, slit, held out wings, and dystroglycan control cardiac lumen formation by establishing nonadherent luminal membranes and regulating cell shape changes. From these data we propose a model for D. melanogaster cardiac lumen formation, which differs, both at a cellular and molecular level, from current models of epithelial tubulogenesis. We suggest that this new example of tube formation may be helpful in studying vertebrate heart tube formation and primary vasculogenesis.

Repulsion by Slit and Roundabout prevents Shotgun/E-cadherin-mediated cell adhesion during Drosophila heart tube lumen formation

During Drosophila melanogaster heart development, a lumen forms between apical surfaces of contralateral cardioblasts (CBs). We show that Slit and its receptor Roundabout (Robo) are required at CB apical domains for lumen formation. Mislocalization of Slit outside the apical domain causes ectopic lumen formation and the mislocalization of cell junction proteins, E-cadherin (E-Cad) and Enabled, without disrupting overall CB cell polarity. Ectopic lumen formation is suppressed in robo mutants, which indicates robo's requirement for this process. Genetic evidence suggests that Robo and Shotgun (Shg)/E-Cad function together in modulating CB adhesion. robo and shg/E-Cad transheterozygotes have lumen defects. In robo loss-of-function or shg/E-Cad gain-of-function embryos, lumen formation is blocked because of inappropriate CB adhesion and an accumulation of E-Cad at the apical membrane. In contrast, shg/E-Cad loss-of-function or robo gain-of-function blocks lumen formation due to a loss of CB adhesion. Our data show that Slit and Robo pathways function in lumen formation as a repulsive signal to antagonize E-Cad-mediated cell adhesion.

The PI 3-kinase and mTOR signaling pathways are important modulators of epithelial tubule formation

Using MDCK cells as a model system, evidence is presented demonstrating that the signaling pathways mammalian target of rapamycin (mTOR) and phosphoinositide 3-kinase (PI 3-kinase) play important roles in the regulation of epithelial tubule formation. Incubation of cells with collagen gel overlays induced early (4-8 h) reorganization of cells (epithelial remodeling) into three-dimensional multicellular tubular structures over 24 h. An MDCK cell line stably expressing the PH domain of Akt, a PI 3-kinase downstream effector, coupled to green fluorescent protein (GFP-Akt-PH) was used to determine the distribution of phosphatidyl inositol-3,4,5-P(3) (PIP(3)), a product of PI 3-kinase. GFP-Akt-PH was associated with lateral membranes in control cells. After incubation with collagen gel overlays, GFP-Akt-PH redistributed into the lamellipodia of migrating cells suggesting that PIP(3) plays a role in epithelial remodeling. Using the small molecule inhibitor LY-294002 that inhibits both mTOR and PI 3-kinase, we demonstrated that kinase activity was required for epithelial remodeling, disruption of cell junctions and subsequent modulation of tubule formation. Since the mTOR signaling pathway is downstream of PI 3-kinase, the effects of rapamycin, a specific mTOR inhibitor, on tubule formation were assessed. Rapamycin did not affect epithelial remodeling or GFP-Akt-PH redistribution but inhibited elongated tubule formation that occurred later (24 h) in morphogenesis. These results were further supported by using RNA interference to down-regulate mTOR and inhibit tubule formation. Our studies demonstrate that PI 3-kinase regulates early epithelial remodeling stages while mTOR modulates latter stages of tubule development.

Myosin II regulates the shape of three-dimensional intestinal epithelial cysts

The development of luminal organs begins with the formation of spherical cysts composed of a single layer of epithelial cells. Using a model three-dimensional cell culture, this study examines the role of a cytoskeletal motor, myosin II, in cyst formation. Caco-2 and SK-CO15 intestinal epithelial cells were embedded into Matrigel, and myosin II was inhibited by blebbistatin or siRNA-mediated knockdown. Whereas control cells formed spherical cysts with a smooth surface, inhibition of myosin II induced the outgrowth of F-actin-rich surface protrusions. The development of these protrusions was abrogated after inhibition of F-actin polymerization or of phospholipase C (PLC) activity, as well as after overexpression of a dominant-negative ADF/cofilin. Surface protrusions were enriched in microtubules and their formation was prevented by microtubule depolymerization. Myosin II inhibition caused a loss of peripheral F-actin bundles and a submembranous extension of cortical microtubules. Our findings suggest that inhibition of myosin II eliminates the cortical F-actin barrier, allowing microtubules to reach and activate PLC at the plasma membrane. PLC-dependent stimulation of ADF/cofilin creates actin-filament barbed ends and promotes the outgrowth of F-actin-rich protrusions. We conclude that myosin II regulates the spherical shape of epithelial cysts by controlling actin polymerization at the cyst surface.

To thine own self be true: self-fusion in single-celled tubes

In this issue of Developmental Cell, Rasmussen et al. (2008) investigate the morphogenesis of the Caenorhabditis elegans pharynx. Their results highlight the usefulness of this system for investigating the molecular mechanisms behind the unusual cell behaviors that underlie the formation of single-celled tubes during animal development.

CRIP homologues maintain apical cytoskeleton to regulate tubule size in C. elegans

Maintenance of the shape and diameter of biological tubules is a critical task in the development and physiology of all metazoan organisms. We have cloned the exc-9 gene of Caenorhabditis elegans, which regulates the diameter of the single-cell excretory canal tubules. exc-9 encodes a homologue of the highly expressed mammalian intestinal LIM-domain protein CRIP, whose function has not previously been determined. A second well-conserved CRIP homologue functions in multiple valves of C. elegans. EXC-9 shows genetic interactions with other EXC proteins, including the EXC-5 guanine exchange factor that regulates CDC-42 activity. EXC-9 and its nematode homologue act in polarized epithelial cells that must maintain great flexibility at their apical surface; our results suggest that CRIPs function to maintain cytoskeletal flexibility at the apical surface.

Antiangiogenic strategies in medulloblastoma: reality or mystery

Medulloblastoma is the most common malignant brain tumor of childhood. Surgery, radiation therapy, and chemotherapy successfully cure many patients, but survivors can suffer long-term toxicities affecting their neurocognitive and growth potential; furthermore, there is no curative therapy in up to 30% of cases, mainly because of our incomplete understanding of many of the underlying molecular and cellular processes. Angiogenesis is a hallmark of the progression of medulloblastoma and, over the last years, investigators have sought to develop effective and less toxic antiangiogenic strategies, including the inhibition or destruction of abnormal blood vessels using either antiangiogenic or vascular disrupting agents. However, the results are conflicting principally because of the complex biology of tumor vasculature and the irregular geometry of the vascular system in real space. In addition, current targets of antiangiogenic therapy, such as vascular endothelial growth factor (VEGF), are thought to be critical for both physiologic and pathologic angiogenesis, and clinical side effects of anti-VEGF therapy are beginning to emerge. We here review the state-of-the-art concerning antiangiogenic targets for medulloblastoma treatment, and discuss the complexity of the vascular system that intrinsically limits the efficacy of current strategies.

COPI vesicle transport is a common requirement for tube expansion in Drosophila

BACKGROUND

Tube expansion defects like stenoses and atresias cause devastating human diseases. Luminal expansion during organogenesis begins to be elucidated in several systems but we still lack a mechanistic view of the process in many organs. The Drosophila tracheal respiratory system provides an amenable model to study tube size regulation. In the trachea, COPII anterograde transport of luminal proteins is required for extracellular matrix assembly and the concurrent tube expansion.

PRINCIPAL FINDINGS

We identified and analyzed Drosophila COPI retrograde transport mutants with narrow tracheal tubes. gammaCOP mutants fail to efficiently secrete luminal components and assemble the luminal chitinous matrix during tracheal tube expansion. Likewise, tube extension is defective in salivary glands, where it also coincides with a failure in the luminal deposition and assembly of a distinct, transient intraluminal matrix. Drosophila gammaCOP colocalizes with cis-Golgi markers and in gammaCOP mutant embryos the ER and Golgi structures are severely disrupted. Analysis of gammaCOP and Sar1 double mutants suggests that bidirectional ER-Golgi traffic maintains the ER and Golgi compartments and is required for secretion and assembly of luminal matrixes during tube expansion.

CONCLUSIONS/SIGNIFICANCE

Our results demonstrate the function of COPI components in organ morphogenesis and highlight the common role of apical secretion and assembly of transient organotypic matrices in tube expansion. Intraluminal matrices have been detected in the notochord of ascidians and zebrafish COPI mutants show defects in notochord expansion. Thus, the programmed deposition and growth of distinct luminal molds may provide distending forces during tube expansion in diverse organs.

Cell-polarity dynamics controls the mechanism of lumen formation in epithelial morphogenesis

Many organs consist of tubes of epithelial cells enclosing a central lumen. How the space of this lumen is generated is a key question in morphogenesis. Two predominant mechanisms of de novo lumen formation have been observed: hollowing and cavitation. In hollowing, the lumen is formed by exocytosis and membrane separation, whereas, in cavitation, the lumen is generated by apoptosis of cells in the middle of the structure [1, 2]. Using MDCK cells in three-dimensional cultures, we found an inverse correlation between polarization efficiency and apoptosis. When cells were grown in collagen, where cells polarized slowly, apoptosis was needed for lumen formation. However, in the presence of Matrigel, which allowed rapid polarization, lumens formed without apoptosis. If polarization in Matrigel was perturbed by blocking formation of the apical surface by RNAi of Cdc42, lumens formed by apoptosis. In a complementary approach, we plated cells at high density so that aggregates formed with little polarity. These aggregates required apoptosis to form lumens, whereas cells plated at low density formed cysts with rapidly polarizing cells and did not need apoptosis to form lumens. The mechanism of lumen formation in the 3D-MDCK model can shift between hollowing and cavitation, depending on cell polarization.

Notch signaling and morphogenesis of single-cell tubes in the C. elegans digestive tract

During organogenesis of the C. elegans digestive system, epithelial cells within a cyst-like primordium develop diverse shapes through largely unknown mechanisms. We here analyze two adjacent, dorsal epithelial cells, called pm8 and vpi1, that remodel their shapes and apical junctions to become donut-shaped, or toroidal, single-cell tubes. pm8 and vpi1 delaminate from the dorsal cyst epithelium and migrate ventrally, across the midline of the cyst, on a transient tract of laminin. pm8 appears to encircle the midline by wrapping around finger-like projections from neighboring cells. Finally, pm8 and vpi1 self-fuse to become toroids by expressing AFF-1 and EFF-1, two fusogens that are each sufficient to promote crossfusion between other cell types. Notch signaling in pm8 induces AFF-1 expression, while simultaneously repressing EFF-1 expression; vpi1 expresses EFF-1 independent of Notch. Thus, the adjacent pm8 and vpi1 cells express different fusogens, allowing them to self-fuse into separate, single-cell tubes while avoiding crossfusion.

Epithelial morphogenesis in embryos: asymmetries, motors and brakes

Epithelial cells play a central role in many embryonic morphogenetic processes, during which they undergo highly coordinated cell shape changes. Here, we review some common principles that have recently emerged through genetic and cellular analyses performed mainly with invertebrate genetic models, focusing on morphogenetic processes involving epithelial sheets. All available data argue that myosin II is the main motor that induces cell shape changes during morphogenesis. We discuss the control of myosin II activity during epithelial morphogenesis, as well as the recently described involvement of microtubules in this process. Finally, we examine how forces unleashed by myosin II can be measured, how embryos use specific brakes to control molecular motors and the potential input of mechano-sensation in morphogenesis.

Crosstalk Between Vascular Endothelial Growth Factor, Notch, and Transforming Growth Factor-β in Vascular Morphogenesis

Vascular morphogenesis encompasses a temporally regulated set of morphological changes that endothelial cells undergo to generate a network of interconnected tubules. Such a complex process inevitably involves multiple cell signaling pathways that must be tightly coordinated in time and space. The formation of a new capillary involves endothelial cell activation, migration, alignment, proliferation, tube formation, branching, anastomosis, and maturation of intercellular junctions and the surrounding basement membrane. Each of these stages is either known or suspected to fall under the influence of the vascular endothelial growth factor, notch, and transforming growth factor-β/bone morphogenetic protein signaling pathways. Vascular endothelial growth factor is essential for initiation of angiogenic sprouting, and also regulates migration of capillary tip cells, proliferation of trunk cells, and gene expression in both. Notch has been implicated in the regulation of cell fate decisions in the vasculature, especially the choice between arterial and venular endothelial cells, and between tip and trunk cell phenotype. Transforming growth factor-β regulates cell migration and proliferation, as well as matrix synthesis. In this review, we emphasize how crosstalk between these pathways is essential for proper patterning of the vasculature and offer a transcriptional oscillator model to explain how these pathways might interact to generate new tip cells during retinal angiogenesis.

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.

A conserved role of the VEGF pathway in angiogenesis of an ectodermally-derived vasculature

Angiogenesis, the growth and remodeling of a vascular network, is an essential process during development, growth and disease. Here we studied the role of the vascular endothelial growth factor receptor (VEGFR) in experimentally-induced angiogenesis in the colonial ascidian Botryllus schlosseri (Tunicata, Ascidiacea). The circulatory system of B. schlosseri is composed of two distinct, but interconnected regions: a plot of sinuses and lacunae which line the body, and a transparent, macroscopic extracorporeal vascular network. The vessels of the extracorporeal vasculature are morphologically inverted in comparison to the vasculature in vertebrates: they consist of a single layer of ectodermally-derived cells with the basal lamina lining the lumen of the vessel. We found that when the peripheral circulatory system of a colony is surgically removed, it can completely regenerate within 24 to 48 h and this regeneration is dependent on proper function of the VEGF pathway: siRNA-mediated knockdown of the VEGFR blocked vascular regeneration, and interfered with vascular homeostasis. In addition, a small molecule, the VEGFR kinase inhibitor PTK787/ZK222584, phenocopied the siRNA knockdown in a reversible manner. Despite the disparate germ layer origins and morphology of the vasculature, the developmental program of branching morphogenesis during angiogenesis is controlled by similar molecular mechanisms, suggesting that the function of the VEGF pathway may be co-opted during the regeneration of an ectoderm-derived tubular structure.

Regulation of cell polarity during epithelial morphogenesis

Epithelial cells have an apical surface facing a lumen or outside of the organism, and a basolateral surface facing other cells and extracellular matrix. The identity of the apical surface is determined by phosphatidylinositol 4,5-bisphosphate, while phosphatidylinositol 3,4,5-trisphophosphate determines the identity of the basolateral surface. The Par3/Par6/atypical protein kinase C complex, as well as the Crumbs and Scribble complexes, controls epithelial polarity. Par4 and AMP kinase regulate polarity during conditions of energy depletion. Lumens are formed in hollow cysts and tubules by fusions of apical vesicles, such as the vacuolar apical compartment, with the plasma membrane. The polarity of individual cells is oriented and coordinated with other cells by interactions with the extracellular matrix.

The Drosophila F-box protein Archipelago controls levels of the Trachealess transcription factor in the embryonic tracheal system

The archipelago gene (ago) encodes the F-box specificity subunit of an SCF(skp-cullin-f box) ubiquitin ligase that inhibits cell proliferation in Drosophila melanogaster and suppresses tumorigenesis in mammals. ago limits mitotic activity by targeting cell cycle and cell growth proteins for ubiquitin-dependent degradation, but the diverse developmental roles of other F-box proteins suggests that it is likely to have additional protein targets. Here we show that ago is required for the post-mitotic shaping of the Drosophila embryonic tracheal system, and that it acts in this tissue by targeting the Trachealess (Trh) protein, a conserved bHLH-PAS transcription factor. ago restricts Trh levels in vivo and antagonizes transcription of the breathless FGF receptor, a known target of Trh in the tracheal system. At a molecular level, the Ago protein binds Trh and is required for proteasome-dependent elimination of Trh in response to expression of the Dysfusion protein. ago mutations that elevate Trh levels in vivo are defective in binding forms of Trh found in Dysfusion-positive cells. These data identify a novel function for the ago ubiquitin-ligase in tracheal morphogenesis via Trh and its target breathless, and suggest that ago has distinct functions in mitotic and post-mitotic cells that influence its role in development and disease.

Multiple cardiovascular defects caused by the absence of alternatively spliced segments of fibronectin

Alternatively spliced variants of fibronectin (FN) containing exons EIIIA and EIIIB are expressed around newly forming vessels in development and disease but are downregulated in mature vasculature. The sequences and patterns of expression of these splice variants are highly conserved among vertebrates, suggestive of their biological importance; however the functions of EIIIA and EIIIB-containing FNs are unknown. To understand the role(s) of these splice variants, we deleted both EIIIA and EIIIB exons from the FN gene and observed embryonic lethality with incomplete penetrance by embryonic day 10.5. Deletion of both EIIIA and EIIIB exons did not affect synthesis or cell surface deposition of FN, indicating that embryonic lethality was due specifically to the absence of EIIIA and EIIIB exons from FN. EIIIA/EIIIB double-null embryos displayed multiple embryonic cardiovascular defects, including vascular hemorrhage, failure of remodeling embryonic and yolk sac vasculature, defective placental angiogenesis and heart defects. In addition, we observed defects in coverage and association with dorsal aortae of alpha-smooth-muscle-actin-positive cells. Our studies indicate that the presence or absence of EIIIA and EIIIB exons alters the function of FN and demonstrate the requirement for these alternatively spliced exons in cardiovascular development.

Fractal dimension as a quantitator of the microvasculature of normal and adenomatous pituitary tissue

It is well known that angiogenesis is a complex process that accompanies neoplastic growth, but pituitary tumours are less vascularized than normal pituitary glands. Several analytical methods aimed at quantifying the vascular system in two-dimensional histological sections have been proposed, with very discordant results. In this study we investigated the non-Euclidean geometrical complexity of the two-dimensional microvasculature of normal pituitary glands and pituitary adenomas by quantifying the surface fractal dimension that measures its space-filling property. We found a statistical significant difference between the mean vascular surface fractal dimension estimated in normal versus adenomatous tissues (P = 0.01), normal versus secreting adenomatous tissues (P = 0.0003), and normal versus non-secreting adenomatous tissues (P = 0.047), whereas the difference between the secreting and non-secreting adenomatous tissues was not statistically significant. This study provides the first demonstration that fractal dimension is an objective and valid quantitator of the two-dimensional geometrical complexity of the pituitary gland microvascular network in physiological and pathological states. Further studies are needed to compare the vascular surface fractal dimension estimates in different subtypes of pituitary tumours and correlate them with clinical parameters in order to evaluate whether the distribution pattern of vascular growth is related to a particular state of the pituitary gland.

Wrapping it up: the cell biology of myelination

During nervous system development, oligodendroglia in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS) synthesise large amounts of specific proteins and lipids to generate myelin, a specialised membrane that spirally ensheathes axons and facilitates fast conduction of the action potential. Myelination is initiated after glial processes have attached to the axon and polarisation of the plasma membrane has been triggered. Myelin assembly is a multi-step process that occurs in spatially distinct regions of the cell. We propose that assembly of myelin proteins and lipids starts during their transport through the biosynthetic pathway and continues at the plasma membrane aided by myelin-basic protein (MBP). These sequential processes create the special lipid and protein composition necessary for myelin to perform its insulating function during nerve conduction.

Tramtrack regulates different morphogenetic events duringDrosophilatracheal development

Tramtrack (Ttk) is a widely expressed transcription factor, the function of which has been analysed in different adult and embryonic tissues in Drosophila. So far, the described roles of Ttk have been mainly related to cell fate specification, cell proliferation and cell cycle regulation. Using the tracheal system of Drosophila as a morphogenetic model, we have undertaken a detailed analysis of Ttk function. Ttk is autonomously and non-autonomously required during embryonic tracheal formation. Remarkably, besides a role in the specification of different tracheal cell identities, we have found that Ttk is directly involved and required for different cellular responses and morphogenetic events. In particular, Ttk appears to be a new positive regulator of tracheal cell intercalation. Analysis of this process in ttk mutants has unveiled cell shape changes as a key requirement for intercalation and has identified Ttk as a novel regulator of its progression. Moreover, we define Ttk as the first identified regulator of intracellular lumen formation and show that it is autonomously involved in the control of tracheal tube size by regulating septate junction activity and cuticle formation. In summary, the involvement of Ttk in different steps of tube morphogenesis identifies it as a key player in tracheal development.

Sequential pulses of apical epithelial secretion and endocytosis drive airway maturation in Drosophila

The development of air-filled respiratory organs is crucial for survival at birth. We used a combination of live imaging and genetic analysis to dissect respiratory organ maturation in the embryonic Drosophila trachea. We found that tracheal tube maturation entails three precise epithelial transitions. Initially, a secretion burst deposits proteins into the lumen. Solid luminal material is then rapidly cleared from the tubes, and shortly thereafter liquid is removed. To elucidate the cellular mechanisms behind these transitions, we identified gas-filling-deficient mutants showing narrow or protein-clogged tubes. These mutations either disrupt endoplasmatic reticulum-to-Golgi vesicle transport or endocytosis. First, Sar1 is required for protein secretion, luminal matrix assembly, and diametric tube expansion. Subsequently, a sharp pulse of Rab5-dependent endocytic activity rapidly internalizes and clears luminal contents. The coordination of luminal matrix secretion and endocytosis may be a general mechanism in tubular organ morphogenesis and maturation.

Failure of epithelial tube maintenance causes hydrocephalus and renal cysts in Dlg5-/- mice

Epithelial tubes represent fundamental building blocks of metazoan organisms; however, the mechanisms responsible for their formation and maintenance are not well understood. Here, we show that the evolutionarily conserved coiled-coil MAGUK protein Dlg5 is required for epithelial tube maintenance in mammalian brain and kidneys. We demonstrate that Dlg5(-/-) mice develop fully penetrant hydrocephalus and kidney cysts caused by a deficiency in membrane delivery of cadherin-catenin adhesion complexes and loss of cell polarity. Dlg5 travels with cadherin-containing vesicles and binds to syntaxin 4, a t-SNARE protein that regulates fusion of transport vesicles with the lateral membrane domain. We propose that Dlg5 functions in plasma membrane delivery of cadherins by linking cadherin-containing transport vesicles with the t-SNARE targeting complex. These findings show that Dlg5 is causally involved in hydrocephalus and renal cysts and reveal that targeted membrane delivery of cadherin-catenin adhesion complexes is critical for cell polarity and epithelial tube maintenance.

The tubulogenic effect of aldosterone is attributed to intact binding and intracellular response of the mineralocorticoid receptor

Little is known about the extra- and intracellular stimuli inducing renal stem/progenitor cells to develop into three-dimensionally structured tubules. To study this specific development in a controlled environment, we used an advanced culture technique. Embryonic tissue derived from neonatal rabbit kidney was placed in a perfusion culture container at the interface of an artificial interstitium made of a polyester fleece. Culture was carried out in chemically defined Iscove’s Modified Dulbecco’s Medium (IMDM) for 13 days. Development of tubules was histochemically detected on cryosections labeled with Soybean Agglutinin (SBA). The experiments showed that aldosterone exerts a specific tubulogenic effect. Application of aldosterone (1 × 10−7 M) raised numerous SBA-labeled tubules, while in the absence of the steroid hormone the development of tubules was lacking. Specificity of hormone action was analyzed by the use of aldosterone antagonists. Administration of spironolactone (1 × 10−4 M) and canrenoate (1 × 10−5 M) completely inhibited the development of tubules. Finally, disrupting the intracellular molecular complex of the mineralocorticoid receptor (MR) and heat shock proteins by geldanamycin (2 μg/ml) prevented the development of tubules. Our results suggest that the tubulogenic effect induced by aldosterone is attributed to both hormone binding and an undisturbed intracellular response of the MR.

Application of proteomics to ecology and population biology

Proteomics is a relatively new scientific discipline that merges protein biochemistry, genome biology and bioinformatics to determine the spatial and temporal expression of proteins in cells, tissues and whole organisms. There has been very little application of proteomics to the fields of behavioral genetics, evolution, ecology and population dynamics, and has only recently been effectively applied to the closely allied fields of molecular evolution and genetics. However, there exists considerable potential for proteomics to impact in areas related to functional ecology; this review will introduce the general concepts and methodologies that define the field of proteomics and compare and contrast the advantages and disadvantages with other methods. Examples of how proteomics can aid, complement and indeed extend the study of functional ecology will be discussed including the main tool of ecological studies, population genetics with an emphasis on metapopulation structure analysis. Because proteomic analyses provide a direct measure of gene expression, it obviates some of the limitations associated with other genomic approaches, such as microarray and EST analyses. Likewise, in conjunction with associated bioinformatics and molecular evolutionary tools, proteomics can provide the foundation of a systems-level integration approach that can enhance ecological studies. It can be envisioned that proteomics will provide important new information on issues specific to metapopulation biology and adaptive processes in nature. A specific example of the application of proteomics to sperm ageing is provided to illustrate the potential utility of the approach.

Molecular regulation of angiogenesis and lymphangiogenesis

Blood vessels and lymphatic vessels form extensive networks that are essential for the transport of fluids, gases, macromolecules and cells within the large and complex bodies of vertebrates. Both of these vascular structures are lined with endothelial cells that integrate functionally into different organs, acquire tissue-specific specialization and retain plasticity; thereby, they permit growth during tissue repair or in disease settings. The angiogenic growth of blood vessels and lymphatic vessels coordinates several biological processes such as cell proliferation, guided migration, differentiation and cell-cell communication.

Genetic control of single lumen formation in the zebrafish gut

Most organs consist of networks of interconnected tubes that serve as conduits to transport fluid and cells and act as physiological barriers between compartments. Biological tubes are assembled through very diverse developmental processes that generate structures of different shapes and sizes. Nevertheless, all biological tubes invariably possess one single lumen. The mechanisms responsible for single lumen specification are not known. Here we show that zebrafish mutants for the MODY5 and familial GCKD gene tcf2 (also known as vhnf1) fail to specify a single lumen in their gut tube and instead develop multiple lumens. We show that Tcf2 controls single lumen formation by regulating claudin15 and Na+/K+-ATPase expression. Our in vivo and in vitro results indicate that Claudin15 functions in paracellular ion transport to specify single lumen formation. This work shows that single lumen formation is genetically controlled and appears to be driven by the accumulation of fluid.

A genetic mosaic analysis with a repressible cell marker screen to identify genes involved in tracheal cell migration during Drosophila air sac morphogenesis

Branching morphogenesis of the Drosophila tracheal system relies on the fibroblast growth factor receptor (FGFR) signaling pathway. The Drosophila FGF ligand Branchless (Bnl) and the FGFR Breathless (Btl/FGFR) are required for cell migration during the establishment of the interconnected network of tracheal tubes. However, due to an important maternal contribution of members of the FGFR pathway in the oocyte, a thorough genetic dissection of the role of components of the FGFR signaling cascade in tracheal cell migration is impossible in the embryo. To bypass this shortcoming, we studied tracheal cell migration in the dorsal air sac primordium, a structure that forms during late larval development. Using a mosaic analysis with a repressible cell marker (MARCM) clone approach in mosaic animals, combined with an ethyl methanesulfonate (EMS)-mutagenesis screen of the left arm of the second chromosome, we identified novel genes implicated in cell migration. We screened 1123 mutagenized lines and identified 47 lines displaying tracheal cell migration defects in the air sac primordium. Using complementation analyses based on lethality, mutations in 20 of these lines were genetically mapped to specific genomic areas. Three of the mutants were mapped to either the Mhc or the stam complementation groups. Further experiments confirmed that these genes are required for cell migration in the tracheal air sac primordium.

Cellular mechanisms of Müllerian duct formation in the mouse

Regardless of their sex chromosome karyotype, amniotes develop two pairs of genital ducts, the Wolffian and Müllerian ducts. As the Müllerian duct forms, its growing tip is intimately associated with the Wolffian duct as it elongates to the urogenital sinus. Previous studies have shown that the presence of the Wolffian duct is required for the development and maintenance of the Müllerian duct. The Müllerian duct is known to form by invagination of the coelomic epithelium, but the mechanism for its elongation to the urogenital sinus remains to be defined. Using genetic fate mapping, we demonstrate that the Wolffian duct does not contribute cells to the Müllerian duct. Experimental embryological manipulations and molecular studies show that precursor cells at the caudal tip of the Müllerian duct proliferate to deposit a cord of cells along the length of the urogenital ridge. Furthermore, immunohistochemical analysis reveals that the cells of the developing Müllerian duct are mesoepithelial when deposited, and subsequently differentiate into an epithelial tube and eventually the female reproductive tract. Our studies define cellular and molecular mechanisms for Müllerian duct formation.

Two T-box genes play independent and cooperative roles to regulate morphogenesis of ciliated Kupffer's vesicle in zebrafish

The brain, heart and gastro-intestinal tract develop distinct left-right (LR) asymmetries. Asymmetric cilia-dependent fluid flow in the embryonic node in mouse, Kupffer's vesicle in zebrafish, notochordal plate in rabbit and gastrocoel roof plate in frog appears to be a conserved mechanism that directs LR asymmetric gene expression and establishes the orientation of organ asymmetry. However, the cellular processes and genetic pathways that control the formation of these essential ciliated structures are unknown. In zebrafish, migratory dorsal forerunner cells (DFCs) give rise to Kupffer's vesicle (KV), a ciliated epithelial sheet that forms a lumen and generates fluid flow. Using the epithelial marker atypical Protein Kinase C (aPKC) and other markers to analyze DFCs and KV cells, we describe a multi-step process by which DFCs form a functional KV. Using mutants and morpholinos, we show that two T-box transcription factors-No tail (Ntl)/Brachyury and Tbx16/Spadetail-cooperatively regulate an early step of DFC mesenchyme to epithelial transition (MET) and KV cell specification. Subsequently, each transcription factor independently controls a distinct step in KV formation: Tbx16 regulates apical clustering of KV cells and Ntl is necessary for KV lumen formation. By targeting morpholinos to DFCs, we show that these cell autonomous functions in KV morphogenesis are necessary for LR patterning throughout the embryo.

Geometry of human vascular system: is it an obstacle for quantifying antiangiogenic therapies?

It is now recognized that all human natural and diseased anatomic systems are characterized by irregular shapes and very complex behaviors. In geometrical terms, tumor vascularity (which is the result of a nonlinear dynamic process called angiogenesis) is an archetypal anatomic system that irregularly fills a 3-dimensional Euclidean space. This characteristic, together with the highly variable nature of vessel shapes and surfaces, leads to considerable spatial and temporal heterogeneity in the delivery of oxygen, nutrients, and drugs, and the removal of metabolites. Although these biologic features have been well established, the quantitative analysis of neovascularity in 2-dimensional histologic sections still fails to view its architecture as a non-Euclidean geometrical object, thus allowing errors in visual interpretation and discordant results concerning the same tumor from different laboratories. We discuss here the tumor-induced vascular system as a fractal object, and what changes this new way of observing may bring to the quantification of effective antiangiogenic therapies.

Non-classical export of epimorphin and its adhesion to alphav-integrin in regulation of epithelial morphogenesis

Epimorphin (also known as syntaxin 2) acts as an epithelial morphogen when secreted by stromal cells of the mammary gland, lung, liver, colon, pancreas and other tissues, but the same molecule functions within the cell to mediate membrane fusion. How this molecule, which lacks a signal sequence and contains a transmembrane domain at the C-terminus, translocates across the plasma membrane and is secreted to become a morphogen, and how it initiates morphogenic events is not clear. Here, we show that epimorphin is secreted through a non-classical mechanism, similar to that previously described for secretion of the leaderless protein FGF1, and we identify the key molecular elements responsible for translocation and secretion from the cell. We also show that secreted epimorphin binds to alphav-integrin-containing receptors on target epithelial cells, leading to activation of specific downstream signaling pathways and induction of epithelial morphogenesis. These findings provide key insight into how epimorphin functions as an epithelial morphogen.

Cilia containing 9 + 2 structures grown from immortalized cells

Cilia depend on their highly differentiated structure, a 9 + 2 arrangement, to remove particles from the lung and to transport reproductive cells. Immortalized cells could potentially be of great use in cilia research. Immortalization of cells with cilia structure containing the 9 + 2 arrangement might be able to generate cell lines with such cilia structure. However, whether immortalized cells can retain such a highly differentiated structure remains unclear. Here we demonstrate that (1) using E1a gene transfection, tracheal cells are immortalized; (2) interestingly, in a gel culture the immortalized cells form spherical aggregations within which a lumen is developed; and (3) surprisingly, inside the aggregation, cilia containing a 9 + 2 arrangement grow from the cell's apical pole and protrude into the lumen. These results may influence future research in many areas such as understanding the mechanisms of cilia differentiation, cilia generation in other existing cell lines, cilia disorders, generation of other highly differentiated structures besides cilia using the gel culture, immortalization of other ciliated cells with the E1a gene, development of cilia motile function, and establishment of a research model to provide uniform ciliated cells.

ARF6-dependent activation of ERK and Rac1 modulates epithelial tubule development

Tubules are the building blocks of epithelial organs and form in response to cues derived from morphogens such as hepatocyte growth factor (HGF). Relatively little is known about signaling pathways that orchestrate the cellular behaviors that constitute tubule development. Here, using three-dimensional cell cultures of Madin-Darby canine kidney cells, we show that the ARF6 GTPase is a critical determinant of tubule initiation in response to HGF. ARF6 is transiently activated during tubulogenesis and perturbing the ARF6 GTP/GDP cycle by inducible expression of ARF6 mutants defective in GTP binding or hydrolysis, inhibits the development of mature tubules. Further, we show that activation of ARF6 is necessary and sufficient to initiate tubule extension. The effect of ARF6 on tubule initiation is two-fold. First, ARF6 regulates the subcellular distribution of the GTPase, Rac1, to tubule extensions. Second, ARF6-induced ERK activation regulates Rac1 activation during tubule initiation through the expression of the receptor for urokinase type plasminogen activator. Thus, we have identified a cellular apparatus downstream of ARF6 activation, which regulates membrane and cytoskeleton remodeling necessary for the early stages of tubule development.

PTEN-mediated apical segregation of phosphoinositides controls epithelial morphogenesis through Cdc42

Formation of the apical surface and lumen is a fundamental, yet poorly understood, step in epithelial organ development. We show that PTEN localizes to the apical plasma membrane during epithelial morphogenesis to mediate the enrichment of PtdIns(4,5)P2 at this domain during cyst development in three-dimensional culture. Ectopic PtdIns(4,5)P2 at the basolateral surface causes apical proteins to relocalize to the basolateral surface. Annexin 2 (Anx2) binds PtdIns(4,5)P2 and is recruited to the apical surface. Anx2 binds Cdc42, recruiting it to the apical surface. Cdc42 recruits aPKC to the apical surface. Loss of function of PTEN, Anx2, Cdc42, or aPKC prevents normal development of the apical surface and lumen. We conclude that the mechanism of PTEN, PtdIns(4,5)P2, Anx2, Cdc42, and aPKC controls apical plasma membrane and lumen formation.

Phosphoinositides specify polarity during epithelial organ development

The molecular mechanisms that integrate cellular polarity with tissue architecture during epithelial morphogenesis are poorly understood. Using a three-dimensional model of epithelial morphogenesis, report that the phosphatase PTEN and phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P2] regulate the GTPase Cdc42 and the kinase aPKC to generate the apical plasma membrane domain and maintain apical-basolateral polarity.

Formation of cysts by alveolar type II cells in three-dimensional culture reveals a novel mechanism for epithelial morphogenesis

Many organs consist of a hollow cavity surrounded by a monolayer of epithelial cells. Despite their common structure, such organs form by diverse morphogenetic processes. Three-dimensional culture systems have been useful in analyzing the events. Most processes require a combination of cell proliferation and cell death to produce a hollow cavity. Here, we describe a new three-dimensional culture system in which primary human lung alveolar type II cells formed hollow epithelial cysts by a novel process. Individual cells moved, collided, and formed alveolar-like cysts without appreciable proliferation or apoptosis. The alveolar-like cysts consisted of a polarized monolayer of differentiated alveolar type II cells, which secreted surfactant into the central lumen. Blockage of beta1 integrin did not alter cell movement or collision, but it greatly reduced adhesion of cells after collision and subsequent formation of alveolar-like cysts. Treatment of preformed alveolar-like cysts with forskolin increased their diameter, possibly due to stimulation of fluid secretion into the lumen. We conclude that epithelial differentiation and cyst formation can occur without appreciable proliferation or apoptosis.

Low concentrations of transforming growth factor-beta-1 induce tubulogenesis in cultured mammary epithelial cells

Background

Formation of branching tubes is a fundamental step in the development of glandular organs. To identify extracellular cues that orchestrate epithelial tubulogenesis, we employed an in vitro assay in which EpH4-J3B1A mammary epithelial cells form spheroidal cysts when grown in collagen gels under serum-free conditions, but form branching tubules in the presence of fetal calf serum (FCS).

Results

Initial experiments showed that the tubulogenesis-inducing activity of FCS was markedly increased by heating (70°C) or transient acidification to pH3. We therefore hypothesized that the tubulogenic agent was transforming growth factor-beta (TGF-beta), a cytokine that is present in serum in latent form and can be activated by heat or acid treatment. We found indeed that the tubulogenic activity of acidified FCS is abrogated by addition of either SB-431542, a selective inhibitor of the TGF-beta type I receptor, or a neutralizing antibody to TGF-beta-1. On the other hand, addition of low concentrations (20–100 pg/ml) of exogenous TGF-beta-1 recapitulated the effect of acidified FCS in inducing morphogenesis of hollow tubes. In contrast, higher concentrations of TGF-beta-1 induced the formation of thin cellular cords devoid of a detectable lumen. To gain insight into the mechanisms underlying TGF-beta-1-induced tube formation, we assessed the potential role of matrix metalloproteinases (MMPs). By western blot and gelatin zymography, we observed a dose-dependent increase in MMP-9 upon TGF-beta-1 treatment. Tube formation was suppressed by a synthetic broad-spectrum metalloproteinase inhibitor, by recombinant tissue inhibitor of metalloproteinases-2 (TIMP-2) and by a selective inhibitor of MMP-9, indicating that this morphogenetic process requires the activity of MMP-9.

Conclusion

Altogether, our results provide evidence that, at low concentrations, TGF-beta-1 promotes MMP-dependent branching tubulogenesis by mammary epithelial cells in vitro, and suggest that it plays a similar role during mammary gland development in vivo.

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.