Patterning mechanisms of branched organs

Coherent angular motion in the establishment of multicellular architecture of glandular tissues

Glandular tissues form ducts (tubes) and acini (spheres) in multicellular organisms. This process is best demonstrated in the organization of the ductal tree of the mammary gland and in 3D models of morphogenesis in culture. Here, we asked a fundamental question: How do single adult epithelial cells generate polarized acini when placed in a surrogate basement membrane 3D gel? Using human breast epithelial cells from either reduction mammoplasty or nonmalignant breast cell lines, we observed a unique cellular movement where single cells undergo multiple rotations and then maintain it cohesively as they divide to assemble into acini. This coherent angular motion (CAMo) was observed in both primary cells and breast cell lines. If CAMo was disrupted, the final geometry was not a sphere. The malignant counterparts of the human breast cell lines in 3D were randomly motile, did not display CAMo, and did not form spheres. Upon "phenotypic reversion" of malignant cells, both CAMo and spherical architecture were restored. We show that cell-cell adhesion and tissue polarity are essential for the formation of acini and link the functional relevance of CAMo to the establishment of spherical architecture rather than to multicellular aggregation or growth. We propose that CAMo is an integral step in the formation of the tissue architecture and that its disruption is involved in malignant transformation.

Normal morphogenesis of epithelial tissues and progression of epithelial tumors

Epithelial cells organize into various tissue architectures that largely maintain their structure throughout the life of an organism. For decades, the morphogenesis of epithelial tissues has fascinated scientists at the interface of cell, developmental, and molecular biology. Systems biology offers ways to combine knowledge from these disciplines by building integrative models that are quantitative and predictive. Can such models be useful for gaining a deeper understanding of epithelial morphogenesis? Here, we take inventory of some recurring themes in epithelial morphogenesis that systems approaches could strive to capture. Predictive understanding of morphogenesis at the systems level would prove especially valuable for diseases such as cancer, where epithelial tissue architecture is profoundly disrupted.

The maintenance and regeneration of the planarian excretory system are regulated by EGFR signaling

The maintenance of organs and their regeneration in case of injury are crucial to the survival of all animals. High rates of tissue turnover and nearly unlimited regenerative capabilities make planarian flatworms an ideal system with which to investigate these important processes, yet little is known about the cell biology and anatomy of their organs. Here we focus on the planarian excretory system, which consists of internal protonephridial tubules. We find that these assemble into complex branching patterns with a stereotyped succession of cell types along their length. Organ regeneration is likely to originate from a precursor structure arising in the blastema, which undergoes extensive branching morphogenesis. In an RNAi screen of signaling molecules, we identified an EGF receptor (Smed-EGFR-5) as a crucial regulator of branching morphogenesis and maintenance. Overall, our characterization of the planarian protonephridial system establishes a new paradigm for regenerative organogenesis and provides a platform for exploring its functional and evolutionary homologies with vertebrate excretory systems.

Drosophila syndecan regulates tracheal cell migration by stabilizing Robo levels

Here we identify a new role for Syndecan (Sdc), the only transmembrane heparan sulphate proteoglycan in Drosophila, in tracheal development. Sdc is required cell autonomously for efficient directed migration and fusion of dorsal branch cells, but not for dorsal branch formation per se. The cytoplasmic domain of Sdc is dispensable, indicating that Sdc does not transduce a signal by itself. Although the branch-specific phenotype of sdc mutants resembles those seen in the absence of Slit/Robo2 signalling, genetic interaction experiments indicate that Sdc also helps to suppress Slit/Robo2 signalling. We conclude that Sdc cell autonomously regulates Slit/Robo2 signalling in tracheal cells to guarantee ordered directional migration and branch fusion.

VAB-10 spectraplakin acts in cell and nuclear migration in Caenorhabditis elegans

Cytoskeletal regulation is important in cell migration. The Caenorhabditis elegans gonadal distal tip cells (DTCs) offer a simple model with which to investigate the mechanism of cell migration in organogenesis. Here, we report that one of the spectraplakin isoforms, VAB-10B1, plays an essential role in cell and nuclear migration of DTCs by regulating the actin and microtubule (MT) cytoskeleton. In the vab-10(tk27) mutant, which lacks VAB-10B1, alignment of filamentous (F)-actin and MTs was weakly and severely disorganized, respectively, which resulted in a failure to translocate the DTC nucleus and a premature termination of DTC migration. An MT growing-tip marker, EBP-2-GFP, revealed that polarized outgrowth of MTs towards the nuclei of migrating DTCs was strikingly impaired in tk27 animals. A vab-10 mini-gene encoding only the actin- and MT-binding domains significantly rescued the gonadal defects, suggesting that VAB-10B1 has a role in linking actin and MT filaments. These results suggest that VAB-10B1/spectraplakin regulates the polarized alignment of MTs, possibly by linking F-actin and MTs, which enables normal nuclear translocation and cell migration of DTCs.

Dynamics of salivary gland morphogenesis

Salivary glands form during embryonic development by a complex process that creates compact, highly organized secretory organs with functions essential for oral health. The architecture of these glands is generated by branching morphogenesis, revealed by recent research to involve unexpectedly dynamic cell motility and novel regulatory pathways. Numerous growth factors, extracellular matrix molecules, gene regulatory pathways, and mechanical forces contribute to salivary gland morphogenesis, but local gene regulation and morphological changes appear to play particularly notable roles. Here we review these recent advances and their potential application to salivary gland tissue engineering.

Integrated morphodynamic signalling of the mammary gland

The mammary gland undergoes a spectacular series of changes as it develops, and maintains a remarkable capacity to remodel and regenerate for several decades. Mammary morphogenesis has been investigated for over 100 years, motivated by the dairy industry and cancer biologists. Over the past decade, the gland has emerged as a major model system in its own right for understanding the cell biology of tissue morphogenesis. Multiple signalling pathways from several cell types are orchestrated together with mechanical cues and cell rearrangements to establish the pattern of the mammary gland. The integrated mechanical and molecular pathways that control mammary morphogenesis have implications for the developmental regulation of other epithelial organs.

A perfusion-independent role of blood vessels in determining branching stereotypy of lung airways

Blood vessels have been shown to play perfusion-independent roles in organogenesis. Here, we examined whether blood vessels determine branching stereotypy of the mouse lung airways in which coordinated branching of epithelial and vascular tubes culminates in their co-alignment. Using different ablative strategies to eliminate the lung vasculature, both in vivo and in lung explants, we show that proximity to the vasculature is indeed essential for patterning airway branching. Remarkably, although epithelial branching per se proceeded at a nearly normal rate, branching stereotypy was dramatically perturbed following vascular ablation. Specifically, branching events requiring a rotation to change the branching plane were selectively affected. This was evidenced by either the complete absence or the shallow angle of their projections, with both events contributing to an overall flat lung morphology. Vascular ablation also led to a high frequency of ectopic branching. Regain of vascularization fully rescued arrested airway branching and restored normal lung size and its three-dimensional architecture. This role of the vasculature is independent of perfusion, flow or blood-borne substances. Inhibition of normal branching resulting from vascular loss could be explained in part by perturbing the unique spatial expression pattern of the key branching mediator FGF10 and by misregulated expression of the branching regulators Shh and sprouty2. Together, these findings uncovered a novel role of the vasculature in organogenesis, namely, determining stereotypy of epithelial branching morphogenesis.

Distinct cytoskeleton populations and extensive crosstalk control Ciona notochord tubulogenesis

Cell elongation is a fundamental process that allows cells and tissues to adopt new shapes and functions. During notochord tubulogenesis in the ascidian Ciona intestinalis, a dramatic elongation of individual cells takes place that lengthens the notochord and, consequently, the entire embryo. We find a novel dynamic actin- and non-muscle myosin II-containing constriction midway along the anteroposterior aspect of each notochord cell during this process. Both actin polymerization and myosin II activity are required for the constriction and cell elongation. Discontinuous localization of myosin II in the constriction indicates that the actomyosin network produces local contractions along the circumference. This reveals basal constriction by the actomyosin network as a novel mechanism for cell elongation. Following elongation, the notochord cells undergo a mesenchymal-epithelial transition and form two apical domains at opposite ends. Extracellular lumens then form at the apical surfaces. We show that cortical actin and Ciona ezrin/radixin/moesin (ERM) are essential for lumen formation and that a polarized network of microtubules, which contributes to lumen development, forms in an actin-dependent manner at the apical cortex. Later in notochord tubulogenesis, when notochord cells initiate a bi-directional crawling movement on the notochordal sheath, the microtubule network rotates 90° and becomes organized as parallel bundles extending towards the leading edges of tractive lamellipodia. This process is required for the correct organization of actin-based protrusions and subsequent lumen coalescence. In summary, we establish the contribution of the actomyosin and microtubule networks to notochord tubulogenesis and reveal extensive crosstalk and regulation between these two cytoskeleton components.

Mammary gland reprogramming: metalloproteinases couple form with function

The adult mammary structure provides for the rapid growth, development, and immunological protection of the live-born young of mammals through its production of milk. The dynamic remodeling of the branched epithelial structure of the mammary gland in response to physiological stimuli that allow its programmed branching morphogenesis at puberty, cyclical turnover during the reproductive cycle, differentiation into a secretory organ at parturition, postlactational involution, and ultimately, regression with age is critical for these processes. Extracellular metalloproteinases are essential for the remodeling programs that operate in the tissue microenvironment at the interface of the epithelium and the stroma, coupling form with function. Deregulated proteolytic activity drives the transition of a physiological mammary microenvironment into a tumor microenvironment, facilitating malignant transformation.

Mesenchymal miR-21 regulates branching morphogenesis in murine submandibular gland in vitro

Branching morphogenesis in murine submandibular glands (SMG) is regulated by growth factors, extracellular matrix (ECM) and many biological processes through interactions between the epithelium and the mesenchyme. MicroRNAs (miRNAs) are a set of small, non-protein-coding RNAs that regulate gene expression at the post-transcriptional level. We hypothesized that branching morphogenesis is partly regulated by miRNAs. Forty-four miRNAs and novel miRNA candidates were detected in SMG at embryonic day 13 by a cloning method combined with Argonaute-2 immunoprecipitation. MicroRNA21 (miR-21) expression in the mesenchyme was up-regulated and accelerated by epidermal growth factor, which is known to enhance branching morphogenesis in vitro. Down-regulation of miR-21 in the mesenchyme by locked nucleic acids was associated with a decrease in the number of epithelial buds. Relative quantification of candidates for target genes of miR-21 indicated that two messenger RNAs (for Reck and Pdcd4) were down-regulated in the mesenchyme, where miR-21 expression levels were up-regulated. These results suggest that branching morphogenesis is regulated by miR-21 through gene expression related to ECM degradation in the mesenchyme.

Branch formation during organ development

Invertebrates and vertebrates use branching morphogenesis to build epithelial trees to maximize the surface area of organs within a given volume. Several molecular regulators of branching have recently been discovered, a number of which are conserved across different organs and species. Signals that control branching at the cellular and tissue levels are also starting to emerge, and are rapidly unveiling the physical nature of branch development. Here we discuss the molecular, cellular, and physical processes that govern branch formation, and highlight the major outstanding questions in the field.

Faulty epithelial polarity genes and cancer

Epithelial architecture is formed in tissues and organs when groups of epithelial cells are organized into polarized structures. The epithelial function and integrity as well as signaling across the epithelial layer is orchestrated by apical junctional complexes (AJCs), which are landmarks for PAR/CRUMBS and lateral SCRIB polarity modules and by dynamic interactions of the cells with underlying basement membrane (BM). These highly organized epithelial architectures are demolished in cancer. In all advanced epithelial cancers, malignant cells have lost polarity and connections to the basement membrane and they have become proliferative, motile, and invasive. Clearly, loss of epithelial integrity associates with tumor progression but does it contribute to tumor development? Evidence from studies in Drosophila and recently also in vertebrate models have suggested that even the oncogene-driven enforced cell proliferation can be conditional, dependant on the influence of cell-cell or cell-microenvironment contacts. Therefore, loss of epithelial integrity may not only be an obligate consequence of unscheduled proliferation of malignant cells but instead, malignant epithelial cells may need to acquire capacity to break free from the constraints of integrity to freely and autonomously proliferate. We discuss how epithelial polarity complexes form and regulate epithelial integrity, highlighting the roles of enzymes Rho GTPases, aPKCs, PI3K, and type II transmembrane serine proteases (TTSPs). We also discuss relevance of these pathways to cancer in light of genetic alterations found in human cancers and review molecular pathways and potential pharmacological strategies to revert or selectively eradicate disorganized tumor epithelium.

Epithelial dynamics of pancreatic branching morphogenesis

The mammalian pancreas is a highly branched gland, essential for both digestion and glucose homeostasis. Pancreatic branching, however, is poorly understood, both at the ultrastructural and cellular levels. In this article, we characterize the morphogenesis of pancreatic branches, from gross anatomy to the dynamics of their epithelial organization. We identify trends in pancreatic branch morphology and introduce a novel mechanism for branch formation, which involves transient epithelial stratification and partial loss of cell polarity, changes in cell shape and cell rearrangements, de novo tubulogenesis and epithelial tubule remodeling. In contrast to the classical epithelial budding and tube extension observed in other organs, a pancreatic branch takes shape as a multi-lumen tubular plexus coordinately extends and remodels into a ramifying, single-lumen ductal system. Moreover, our studies identify a role for EphB signaling in epithelial remodeling during pancreatic branching. Overall, these results illustrate distinct, step-wise cellular mechanisms by which pancreatic epithelium shapes itself to create a functional branching organ.

Emergence of matched airway and vascular trees from fractal rules

The bronchial, arterial, and venous trees of the lung are complex interwoven structures. Their geometries are created during fetal development through common processes of branching morphogenesis. Insights from fractal geometry suggest that these extensively arborizing trees may be created through simple recursive rules. Mathematical models of Turing have demonstrated how only a few proteins could interact to direct this branching morphogenesis. Development of the airway and vascular trees could, therefore, be considered an example of emergent behavior as complex structures are created from the interaction of only a few processes. However, unlike inanimate emergent structures, the geometries of the airway and vascular trees are highly stereotyped. This review will integrate the concepts of emergence, fractals, and evolution to demonstrate how the complex branching geometries of the airway and vascular trees are ideally suited for gas exchange in the lung. The review will also speculate on how the heterogeneity of blood flow and ventilation created by the vascular and airway trees is overcome through their coordinated construction during fetal development.

MET signalling: principles and functions in development, organ regeneration and cancer

The MET tyrosine kinase receptor (also known as the HGF receptor) promotes tissue remodelling, which underlies developmental morphogenesis, wound repair, organ homeostasis and cancer metastasis, by integrating growth, survival and migration cues in response to environmental stimuli or cell-autonomous perturbations. The versatility of MET-mediated biological responses is sustained by qualitative and quantitative signal modulation. Qualitative mechanisms include the engagement of dedicated signal transducers and the subcellular compartmentalization of MET signalling pathways, whereas quantitative regulation involves MET partnering with adaptor amplifiers or being degraded through the shedding of its extracellular domain or through intracellular ubiquitylation. Controlled activation of MET signalling can be exploited in regenerative medicine, whereas MET inhibition might slow down tumour progression.

Evidence of interlobular repulsion during branching morphogenesis in mouse salivary glands

We developed a culture method for detecting repulsion among epithelial lobules during branching morphogenesis in mouse submandibular glands. Three epithelia were placed at each vertex of an imaginary triangle apart but near enough to meet with one another if each of them expands radially during the culture period. No repulsion was observed following cultivation with neuregulin 1 and lysophosphatidic acid; the epithelia actively branched and nearly contacted one another in the triangle's center. In contrast, strong repulsion was observed among the epithelia cultured with fibroblast growth factor 1 (FGF1), which exhibited less branching and moved away from the center. The localization of DiI- (1,1', di-octadecyl-3,3,3',3',-tetramethylindo-carbocyanine perchlorate) and BrdU- (5-bromodeoxyuridine) labeled cells in the cultures exposed to FGF1 indicated that the cells were unable to move and proliferate in the center. SB431542, an inhibitor of transforming growth factor-beta (TGFbeta) signaling, was unable to abolish this repulsion, suggesting that TGFbetas will not probably act as repellants in this case.

Cell-matrix interactions in mammary gland development and breast cancer

The mammary gland is an organ that at once gives life to the young, but at the same time poses one of the greatest threats to the mother. Understanding how the tissue develops and functions is of pressing importance in determining how its control mechanisms break down in breast cancer. Here we argue that the interactions between mammary epithelial cells and their extracellular matrix (ECM) are crucial in the development and function of the tissue. Current strategies for treating breast cancer take advantage of our knowledge of the endocrine regulation of breast development, and the emerging role of stromal-epithelial interactions (Fig. 1). Focusing, in addition, on the microenvironmental influences that arise from cell-matrix interactions will open new opportunities for therapeutic intervention. We suggest that ultimately a three-pronged approach targeting endocrine, growth factor, and cell-matrix interactions will provide the best chance of curing the disease.

Tube continued: morphogenesis of the Drosophila tracheal system

The Drosophila respiratory organ (tracheal system) consists of epithelial tubes, the morphogenesis of which is controlled by distinct sets of signaling pathways and transcription factors. The downstream events controlling tube formation and shape are only now beginning to be identified. Here we review recent insight into the communication between neighboring tracheal cells, their interactions with the surrounding matrix, and the impact of these processes on tube morphogenesis. We focus on cell-cell interactions that drive rearrangement of cells within the epithelium and that are essential for maintenance of epithelial integrity, and also on cell-matrix interactions that play key roles in determining and maintaining the size and shape of tube lumens.

Parasympathetic innervation maintains epithelial progenitor cells during salivary organogenesis

The maintenance of a progenitor cell population as a reservoir of undifferentiated cells is required for organ development and regeneration. However, the mechanisms by which epithelial progenitor cells are maintained during organogenesis are poorly understood. We report that removal of the parasympathetic ganglion in mouse explant organ culture decreased the number and morphogenesis of keratin 5-positive epithelial progenitor cells. These effects were rescued with an acetylcholine analog. We demonstrate that acetylcholine signaling, via the muscarinic M1 receptor and epidermal growth factor receptor, increased epithelial morphogenesis and proliferation of the keratin 5-positive progenitor cells. Parasympathetic innervation maintained the epithelial progenitor cell population in an undifferentiated state, which was required for organogenesis. This mechanism for epithelial progenitor cell maintenance may be targeted for organ repair or regeneration.

Endogenous patterns of mechanical stress are required for branching morphogenesis

Spatial patterning of cell behaviors establishes the regional differences within tissues that collectively develop branched organs into their characteristic treelike shapes. Here we show that the pattern of branching morphogenesis of three-dimensional (3D) engineered epithelial tissues is controlled in part by gradients of endogenous mechanical stress. We used microfabrication to build model mammary epithelial tissues of defined geometry that branched in a stereotyped pattern when induced with growth factors. Branches initiated from sites of high mechanical stress within the tissues, as predicted numerically and measured directly using 3D traction force microscopy. Branch sites were defined by activation of focal adhesion kinase (FAK), inhibition of which disrupted morphogenesis. Stress, FAK activation, and branching were all altered by manipulating cellular contractility, matrix stiffness, intercellular cohesion and tissue geometry. These data suggest that the pattern and magnitude of mechanical stress across epithelial tissues cooperate with biochemical signals to specify branching pattern.

Spatial restriction of FGF signaling by a matrix metalloprotease controls branching morphogenesis

FGF signaling is a central regulator of branching morphogenesis processes, such as angiogenesis or the development of branched organs including lung, kidney, and mammary gland. The formation of the air sac during the development of the Drosophila tracheal system is a powerful genetic model to investigate how FGF signaling patterns such emerging structures. This article describes the characterization of the Drosophila matrix metalloprotease Mmp2 as an extracellular inhibitor of FGF morphogenetic function. Mmp2 expression in the developing air sac is controlled by the Drosophila FGF homolog Branchless and then participates in a negative feedback and lateral inhibition mechanism that defines the precise pattern of FGF signaling. The signaling function for MMPs described here may not be limited to branching morphogenesis processes.

Does plasminogen activator inhibitor-1 drive lymphangiogenesis?

The purpose of this study is to explore the function of plasminogen activator inhibitor-1 (PAI-1) during pathological lymphangiogenesis. PAI-1, the main physiological inhibitor of plasminogen activators is involved in pathological angiogenesis at least by controlling extracellular proteolysis and by regulating endothelial cell survival and migration. Protease system's role in lymphangiogenesis is unknown yet. Thus, based on its important pro-angiogenic effect, we hypothesized that PAI-1 may regulate lymphangiogenesis associated at least with metastatic dissemination of cancer cells. To address this issue, we studied the impact of PAI-1 deficiency in various murine models of tumoral lymphangiogenesis. Wild-type PAI-1 proficient mice were used as controls. We provide for the first time evidence that PAI-1 is dispensable for tumoral lymphangiogenesis associated with breast cancers either induced by mammary carcinoma cell injection or spontaneously appearing in transgenic mice expressing the polyomavirus middle T antigen (PymT) under the control of a mouse mammary tumor virus long-terminal repeat promoter (MMTV-LTR). We also investigated inflammation-related lymphatic vessel recruitment by using two inflammatory models. PAI-1 deficiency did neither affect the development of lymphangioma nor burn-induced corneal lymphangiogenesis. These novel data suggest that vascular remodelling associated with lymphangiogenesis and angiogenesis involve different molecular determinants. PAI-1 does not appear as a potential therapeutic target to counteract pathological lymphangiogenesis.

Mechanisms for split localization of Fgf10 expression in early lung development

In early lung development, epithelial tubes (lung buds) intrude into mesenchyme covered with pleural cells (lung border), and form tree-like networks, by means of repeated use of morphogenetic processes: "elongation," "terminal bifurcation," and "lateral budding." When a bud is elongating, a peak of Fgf10 expression is formed in the mesenchyme near the tip; whereas when terminal bifurcation and lateral budding occur, two separate peaks are formed instead. To explain the spatial pattern of Fgf10 expression, we developed a mathematical model for the regulation of Fgf10 expression with geometrical conditions including shapes of the lung buds and the lung border. Different localization patterns of Fgf10 expression can be explained by the geometrical conditions. Fgf10 expression has a single peak when a length between the tip of lung bud and the lung border is large. When the length is small, Fgf10 expression has two peaks, whose location depends on the curvature of lung border.

Mechanism of primitive duct formation in the pancreas and submandibular glands: a role for SDF-1

Background

The exocrine pancreas is composed of a branched network of ducts connected to acini. They are lined by a monolayered epithelium that derives from the endoderm and is surrounded by mesoderm-derived mesenchyme. The morphogenic mechanisms by which the ductal network is established as well as the signaling pathways involved in this process are poorly understood.

Results

By morphological analyzis of wild-type and mutant mouse embryos and using cultured embryonic explants we investigated how epithelial morphogenesis takes place and is regulated by chemokine signaling. Pancreas ontogenesis displayed a sequence of two opposite epithelial transitions. During the first transition, the monolayered and polarized endodermal cells give rise to tissue buds composed of a mass of non polarized epithelial cells. During the second transition the buds reorganize into branched and polarized epithelial monolayers that further differentiate into tubulo-acinar glands. We found that the second epithelial transition is controlled by the chemokine Stromal cell-Derived Factor (SDF)-1. The latter is expressed by the mesenchyme, whereas its receptor CXCR4 is expressed by the epithelium. Reorganization of cultured pancreatic buds into monolayered epithelia was blocked in the presence of AMD3100, a SDF-1 antagonist. Analyzis of sdf1 and cxcr4 knockout embryos at the stage of the second epithelial transition revealed transient defective morphogenesis of the ventral and dorsal pancreas. Reorganization of a globular mass of epithelial cells in polarized monolayers is also observed during submandibular glands development. We found that SDF-1 and CXCR4 are expressed in this organ and that AMD3100 treatment of submandibular gland explants blocks its branching morphogenesis.

Conclusion

In conclusion, our data show that the primitive pancreatic ductal network, which is lined by a monolayered and polarized epithelium, forms by remodeling of a globular mass of non polarized epithelial cells. Our data also suggest that SDF-1 controls the branching morphogenesis of several exocrine tissues.

GATA3 in development and cancer differentiation: cells GATA have it!

There is increasing evidence that the numerous mechanisms that regulate cell differentiation during normal development are also involved in tumorigenesis. In breast cancer, differentiation markers expressed by the primary tumor are routinely profiled to guide clinical decisions. Indeed, numerous studies have shown that the differentiation profile correlates with the metastatic potential of tumors. The transcription factor GATA3 has emerged recently as a strong predictor of clinical outcome in human luminal breast cancer. In the mammary gland, GATA3 is required for luminal epithelial cell differentiation and commitment, and its expression is progressively lost during luminal breast cancer progression as cancer cells acquire a stem cell-like phenotype. Importantly, expression of GATA3 in GATA3-negative, undifferentiated breast carcinoma cells is sufficient to induce tumor differentiation and inhibits tumor dissemination in a mouse model. These findings demonstrate the exquisite ability of a differentiation factor to affect malignant properties, and raise the possibility that GATA3 or its downstream genes could be used in treating luminal breast cancer. This review highlights our recent understanding of GATA3 in both normal mammary development and tumor differentiation.

MT2-MMP-dependent release of collagen IV NC1 domains regulates submandibular gland branching morphogenesis

Proteolysis is essential during branching morphogenesis, but the roles of MT-MMPs and their proteolytic products are not clearly understood. Here, we discover that decreasing MT-MMP activity during submandibular gland branching morphogenesis decreases proliferation and increases collagen IV and MT-MMP expression. Specifically, reducing epithelial MT2-MMP profoundly decreases proliferation and morphogenesis, increases Col4a2 and intracellular accumulation of collagen IV, and decreases the proteolytic release of collagen IV NC1 domains. Importantly, we demonstrate the presence of collagen IV NC1 domains in developing tissue. Furthermore, recombinant collagen IV NC1 domains rescue branching morphogenesis after MT2-siRNA treatment, increasing MT-MMP and proproliferative gene expression via beta1 integrin and PI3K-AKT signaling. Additionally, HBEGF also rescues MT2-siRNA treatment, increasing NC1 domain release, proliferation, and MT2-MMP and Hbegf expression. Our studies provide mechanistic insight into how MT2-MMP-dependent release of bioactive NC1 domains from collagen IV is critical for integrating collagen IV synthesis and proteolysis with epithelial proliferation during branching morphogenesis.

Pancreatic ductal morphogenesis and the Pdx1 homeodomain transcription factor

Embryonic development of the pancreas is marked by an early phase of dramatic morphogenesis, in which pluripotent progenitor cells of the developing pancreatic epithelium give rise to the full array of mature exocrine and endocrine cell types. The genetic determinants of acinar and islet cell lineages are somewhat well defined; however, the molecular mechanisms directing ductal formation and differentiation remain to be elucidated. The complex ductal architecture of the pancreas is established by a reiterative program of progenitor cell expansion and migration known as branching morphogenesis, or tubulogenesis, which proceeds in mouse development concomitantly with peak Pdx1 transcription factor expression. We therefore evaluated Pdx1 expression with respect to lineage-specific markers in embryonic sections of the pancreas spanning this critical period of duct formation and discovered an unexpected population of nonislet Pdx1-positive cells displaying physical traits of branching. We then established a 3D cell culture model of branching morphogenesis using primary pancreatic duct cells and identified a transient surge of Pdx1 expression exclusive to branching cells. From these observations we propose that Pdx1 might be involved temporally in a program of gene expression sufficient to facilitate the biochemical and morphological changes necessary for branching morphogenesis.

Local guidance of emerging vessel sprouts requires soluble Flt-1

Blood vessel networks form via sprouting of endothelial cells from parent vessels. Extrinsic cues guide sprouts after they leave the initiation site, but these cues are likely insufficient to regulate initial outward movement, and many embryonic vessel networks form in the absence of a strong extrinsic gradient. We hypothesized that nascent sprouts are guided by spatial cues produced along their own vessels, and that soluble Flt-1 (sFlt-1) participates in this guidance. Analysis of developing vessels with perturbed flt-1 function revealed misguided emerging sprouts, and transgenic sFlt-1 rescued sprout guidance parameters. sflt-1 activity in endothelial cells immediately adjacent to the emerging sprout significantly improved local sprout guidance. Thus, we propose that a vessel-intrinsic system initially guides emerging sprouts away from the parent vessel, utilizing spatially regulated expression of sFlt-1 in conjunction with exogenous VEGF-A. Local sprout guidance defects are predicted to contribute to vessel dysmorphogenesis during perturbed development and disease.

Morphogenesis of epithelial tubes: Insights into tube formation, elongation, and elaboration

Epithelial tubes are a fundamental tissue across the metazoan phyla and provide an essential functional component of many of the major organs. Recent work in flies and mammals has begun to elucidate the cellular mechanisms driving the formation, elongation, and branching morphogenesis of epithelial tubes during development. Both forward and reverse genetic techniques have begun to identify critical molecular regulators for these processes and have revealed the conserved role of key pathways in regulating the growth and elaboration of tubular networks. In this review, we discuss the developmental programs driving the formation of branched epithelial networks, with specific emphasis on the trachea and salivary gland of Drosophila melanogaster and the mammalian lung, mammary gland, kidney, and salivary gland. We both highlight similarities in the development of these organs and attempt to identify tissue and organism specific strategies. Finally, we briefly consider how our understanding of the regulation of proliferation, apicobasal polarity, and epithelial motility during branching morphogenesis can be applied to understand the pathologic dysregulation of these same processes during metastatic cancer progression.

Dioxin receptor deficiency impairs angiogenesis by a mechanism involving VEGF-A depletion in the endothelium and transforming growth factor-beta overexpression in the stroma

Angiogenesis has key roles in development and in the progression of human diseases such as cancer. Consequently, identifying the novel markers and regulators of angiogenesis is a critical task. The dioxin receptor (AhR) contributes to vascular homeostasis and to the endothelial response to toxins, although the mechanisms involved are largely uncharacterized. Here, we show that AhR-null mice (AhR(-/-)) have impaired angiogenesis in vivo that compromises tumor xenograft growth. Aortic rings emigration experiments and RNA interference indicated that AhR(-/-) endothelial cells failed to branch and to form tube-like structures. Such a phenotype was found to be vascular endothelial growth factor (VEGF)-dependent, as AhR(-/-) aortic endothelial cells (MAECs) secreted lower amounts of active VEGF-A and their treatment with VEGF-A rescued angiogenesis in culture and in vivo. Further, the addition of anti-VEGF antibody to AhR(+/+) MAECs reduced angiogenesis. Treatment under hypoxic conditions with 2-methoxyestradiol suggested that HIF-1alpha modulates endothelial VEGF expression in an AhR-dependent manner. Importantly, AhR-null stromal myofibroblasts produced increased transforming growth factor-beta (TGFbeta) activity, which inhibited angiogenesis in human endothelial cells (HMECs) and AhR(-/-) mice, whereas the co-culture of HMECs with AhR(-/-) myofibroblasts or with their conditioned medium inhibited branching, which was restored by an anti-TGFbeta antibody. Moreover, VEGF and TGFbeta activities cooperated in modulating angiogenesis, as the addition of TGFbeta to AhR(-/-) MAECs further reduced their low basal VEGF-A activity. Thus, AhR modulates angiogenesis through a mechanism requiring VEGF activation in the endothelium and TGFbeta inactivation in the stroma. These data highlight the role of AhR in cardiovascular homeostasis and suggest that this receptor can be a novel regulator of angiogenesis during tumor development.

Cellular and molecular mechanisms underlying the formation of biological tubes

Biological tubes are integral components of many organs. Based on their cellular organization, tubes can be divided into three types: multicellular, unicellular, and intracellular. The mechanisms by which these tubes form during development vary significantly, in many cases even for those sharing a similar final architecture. Here, we present recent advances in studying cellular and molecular aspects of tubulogenesis in different organisms.

Morphogenetic cell movements: diversity from modular mechanical properties

Animal tissue and organ development requires the orchestration of cell movements, including those of interconnected cell groups, termed collective cell movements. Such movements are incredibly diverse. Recent work suggests that two core cellular properties, cell-cell adhesion and contractility, can largely determine geometry, packing, sorting, and rearrangement of epithelial cell layers. Two additional force-generating properties, the ability to generate cell protrusions and cell adhesion to the extracellular matrix, contribute to active motility. These mechanical properties can be regulated independently in cells, suggesting that they can be employed in a combinatorial manner. A small number of properties used in combination could, in principle, generate a diverse array of cell shapes and arrangements and thus orchestrate the varied morphogenetic events observed during metazoan organ development.