Tube morphogenesis: making and shaping biological tubes

Comparative mechanisms of branching morphogenesis in diverse systems

Much progress has been made in recent years toward understanding mechanisms controlling branching morphogenesis, a fundamental aspect of development in a variety of invertebrate and vertebrate organs. To gain a deeper understanding of how branching morphogenesis occurs in the mammary gland, we compare and contrast the cellular and molecular events underlying this process in both invertebrate and vertebrate organs. Thus, in this review, we focus on the common themes that have emerged from such comparative analyses and discuss how they are implemented via a battery of signaling pathways to ensure proper branching morphogenesis in diverse systems.

Tracheal development in the Drosophila brain is constrained by glial cells

The Drosophila brain is tracheated by the cerebral trachea, a branch of the first segmental trachea of the embryo. During larval stages the cerebral trachea splits into several main (primary) branches that grow around the neuropile, forming a perineuropilar tracheal plexus (PNP) at the neuropile surface. Five primary tracheal branches whose spatial relationship to brain compartments is relatively invariant can be distinguished, although the exact trajectories and branching pattern of the brain tracheae are surprisingly variable. Immunohistochemical and electron microscopic studies demonstrate that all brain tracheae grow in direct contact with the glial cell processes that surround the neuropile. To investigate the effect of glia on tracheal development, embryos and larvae lacking glial cells as a result of a genetic mutation or a directed ablation were analyzed. In these animals, the tracheal branching pattern was highly abnormal. In particular, the number of secondary branches entering the central neuropile was increased. Wild-type larvae possess only two central tracheae, typically associated with the mushroom body and the antennocerebral tract. In larvae lacking glial cells, six to ten tracheal branches penetrate the neuropile in a variable pattern. This finding indicates that glia-derived signals constrained tracheal growth in the Drosophila brain and restrict the number of branches entering the neuropile.

Drosophila talin and integrin genes are required for maintenance of tracheal terminal branches and luminal organization

Epithelial tubes that compose many organs are typically long lasting, except under specific developmental and physiological conditions when network remodeling occurs. Although there has been progress elucidating mechanisms of tube formation, little is known of the mechanisms that maintain tubes and destabilize them during network remodeling. Here, we describe Drosophila tendrils mutations that compromise maintenance of tracheal terminal branches, fine gauge tubes formed by tracheal terminal cells that ramify on and adhere tightly to tissues in order to supply them with oxygen. Homozygous tendrils terminal cell clones have fewer terminal branches than normal but individual branches contain multiple convoluted lumens. The phenotype arises late in development: terminal branches bud and form lumens normally early in development, but during larval life lumens become convoluted and mature branches degenerate. Their lumens, however, are retained in the remaining branches, resulting in the distinctive multi-lumen phenotype. Mapping and molecular studies demonstrate that tendrils is allelic to rhea, which encodes Drosophila talin, a large cytoskeletal protein that links integrins to the cytoskeleton. Terminal cells mutant for myospheroid, the major Drosophila beta-integrin, or doubly mutant for multiple edematous wings and inflated alpha-integrins, also show the tendrils phenotype, and localization of myospheroid beta-integrin protein is disrupted in tendrils mutant terminal cells. The results provide evidence that integrin-talin adhesion complexes are necessary to maintain tracheal terminal branches and luminal organization. Similar complexes may stabilize other tubular networks and may be targeted for inactivation during network remodeling events.

EphB2 and EphB4 receptors forward signaling promotes SDF-1-induced endothelial cell chemotaxis and branching remodeling

The complex molecular mechanisms that drive endothelial cell movement and the formation of new vessels are poorly understood and require further investigation. Eph receptor tyrosine kinases and their membrane-anchored ephrin ligands regulate cell movements mostly by cell-cell contact, whereas the G-protein-coupled receptor CXCR4 and its unique SDF-1 chemokine ligand regulate cell movement mostly through soluble gradients. By using biochemical and functional approaches, we investigated how ephrinB and SDF-1 orchestrate endothelial cell movement and morphogenesis into capillary-like structures. We describe how endogenous EphB2 and EphB4 signaling are required for the formation of extracellular matrix-dependent capillary-like structures in primary human endothelial cells. We further demonstrate that EphB2 and EphB4 activation enhance SDF-1-induced signaling and chemotaxis that are also required for extracellular matrix-dependent endothelial cell clustering. These results support a model in which SDF-1 gradients first promote endothelial cell clustering and then EphB2 and EphB4 critically contribute to subsequent cell movement and alignment into cord-like structures. This study reveals a requirement for endogenous Eph signaling in endothelial cell morphogenic processes, uncovers a novel link between EphB forward signaling and SDF-1-induced signaling, and demonstrates a mechanism for cooperative regulation of endothelial cell movement.

Egfr is essential for maintaining epithelial integrity during tracheal remodelling in Drosophila

A fundamental requirement during organogenesis is to preserve tissue integrity to render a mature and functional structure. Many epithelial organs, such as the branched tubular structures, undergo a tremendous process of tissue remodelling to attain their final pattern. The cohesive properties of these tissues need to be finely regulated to promote adhesion yet allow flexibility during extensive tissue remodelling. Here, we report a new role for the Egfr pathway in maintaining epithelial integrity during tracheal development in Drosophila. We show that the integrity-promoting Egfr function is transduced by the ERK-type MAPK pathway, but does not require the downstream transcription factor Pointed. Compromising Egfr signalling, by downregulating different elements of the pathway or by overexpressing the Mkp3 negative regulator, leads to loss of tube integrity, whereas upregulation of the pathway results in increased tissue stiffness. We find that regulation of MAPK pathway activity by Breathless signalling does not impinge on tissue integrity. Egfr effects on tissue integrity correlate with differences in the accumulation of markers for cadherin-based cell-cell adhesion. Accordingly, downregulation of cadherin-based cell-cell adhesion gives rise to tracheal integrity defects. Our results suggest that the Egfr pathway regulates maintenance of tissue integrity, at least in part, through the modulation of cell adhesion. This finding establishes a link between a developmental pathway governing tracheal formation and cell adhesiveness.

Rho kinase, myosin-II, and p42/44 MAPK control extracellular matrix-mediated apical bile canalicular lumen morphogenesis in HepG2 cells

The molecular mechanisms that regulate multicellular architecture and the development of extended apical bile canalicular lumens in hepatocytes are poorly understood. Here, we show that hepatic HepG2 cells cultured on glass coverslips first develop intercellular apical lumens typically formed by a pair of cells. Prolonged cell culture results in extensive organizational changes, including cell clustering, multilayering, and apical lumen morphogenesis. The latter includes the development of large acinar structures and subsequent elongated canalicular lumens that span multiple cells. These morphological changes closely resemble the early organizational pattern during development, regeneration, and neoplasia of the liver and are rapidly induced when cells are cultured on predeposited extracellular matrix (ECM). Inhibition of Rho kinase or its target myosin-II ATPase in cells cultured on glass coverslips mimics the morphogenic response to ECM. Consistently, stimulation of Rho kinase and subsequent myosin-II ATPase activity by lipoxygenase-controlled eicosatetranoic acid metabolism inhibits ECM-mediated cell multilayering and apical lumen morphogenesis but not initial apical lumen formation. Furthermore, apical lumen remodeling but not cell multilayering requires basal p42/44 MAPK activity. Together, the data suggest a role for hepatocyte-derived ECM in the spatial organization of hepatocytes and apical lumen morphogenesis and identify Rho kinase, myosin-II, and MAPK as potentially important players in different aspects of bile canalicular lumen morphogenesis.

Endothelial tubes assemble from intracellular vacuoles in vivo

The formation of epithelial tubes is crucial for the proper development of many different tissues and organs, and occurs by means of a variety of different mechanisms. Morphogenesis of seamless, properly patterned endothelial tubes is essential for the development of a functional vertebrate circulatory system, but the mechanism of vascular lumenization in vivo remains unclear. Evidence dating back more than 100 years has hinted at an important function for endothelial vacuoles in lumen formation. More than 25 years ago, in some of the first endothelial cell culture experiments in vitro, Folkman and Haudenschild described "longitudinal vacuoles" that "appeared to be extruded and connected from one cell to the next", observations confirmed and extended by later studies in vitro showing that intracellular vacuoles arise from integrin-dependent and cdc42/Rac1-dependent pinocytic events downstream of integrin-extracellular-matrix signalling interactions. Despite compelling data supporting a model for the assembly of endothelial tubes in vitro through the formation and fusion of vacuoles, conclusive evidence in vivo has been lacking, primarily because of difficulties associated with imaging the dynamics of subcellular endothelial vacuoles deep within living animals. Here we use high-resolution time-lapse two-photon imaging of transgenic zebrafish to examine how endothelial tubes assemble in vivo, comparing our results with time-lapse imaging of human endothelial-cell tube formation in three-dimensional collagen matrices in vitro. Our results provide strong support for a model in which the formation and intracellular and intercellular fusion of endothelial vacuoles drives vascular lumen formation.

Border of Notch activity establishes a boundary between the two dorsal appendage tube cell types

Boundaries establish and maintain separate populations of cells critical for organ formation. We show that Notch signaling establishes the boundary between two types of post-mitotic epithelial cells, the Rhomboid- and the Broad-positive cells. These cells will undergo morphogenetic movements to generate the two sides of a simple organ, the dorsal appendage tube of the Drosophila egg chamber. The boundary forms due to a difference in Notch levels in adjacent cells. The Notch expression pattern mimics the boundary; Notch levels are high in Rhomboid cells and low in Broad cells. Notch(-) mutant clones generate an ectopic boundary: ectopic Rhomboid cells arise in Notch(+) cells adjacent to the Notch(-) mutant cells but not further away from the clonal border. Pangolin, a component of the Wingless pathway, is required for Broad expression and for rhomboid repression. We further show that Broad represses rhomboid cell autonomously. Our data provide a foundation for understanding how a single row of Rhomboid cells arises adjacent to the Broad cells in the dorsal appendage primordia. Generating a boundary by the Notch pathway might constitute an evolutionarily conserved first step during organ formation in many tissues.

Septate-junction-dependent luminal deposition of chitin deacetylases restricts tube elongation in the Drosophila trachea

The function of tubular epithelial organs like the kidney and lung is critically dependent on the length and diameter of their constituting branches. Genetic analysis of tube size control during Drosophila tracheal development has revealed that epithelial septate junction (SJ) components and the dynamic chitinous luminal matrix coordinate tube growth. However, the underlying molecular mechanisms controlling tube expansion so far remained elusive. Here, we present the analysis of two luminal chitin binding proteins with predicted polysaccharide deacetylase activities (ChLDs). ChLDs are required to assemble the cable-like extracellular matrix (ECM) and restrict tracheal tube elongation. Overexpression of native, but not of mutated, ChLD versions also interferes with the structural integrity of the intraluminal ECM and causes aberrant tube elongation. Whereas ChLD mutants have normal SJ structure and function, the luminal deposition of the ChLD requires intact cellular SJs. This identifies a new molecular function for SJs in the apical secretion of ChLD and positions ChLD downstream of the SJs in tube length control. The deposition of the chitin luminal matrix first promotes and coordinates radial tube expansion. We propose that the subsequent structural modification of chitin by chitin binding deacetylases selectively instructs the termination of tube elongation to the underlying epithelium.

Three-dimensional modeling of mechanical forces in the extracellular matrix during epithelial lumen formation

Mechanical interactions between cells and extracellular matrix (ECM) mediate epithelial cyst formation. This work relies on the combination of numerical modeling with live cell imaging, to piece together a novel nonintrusive method for determining three-dimensional (3D) mechanical forces caused by shape changes of a multicellular aggregate at the early stages of epithelial cyst formation. We analyzed the evolution of Madin-Darby canine kidney cells in 3D cultures using time-lapse microscopy, with type I collagen gel forming the ECM. The evolving 3D interface between the ECM and the cell aggregate was obtained from microscopy images, and the stress on the surface of a proliferating aggregate and in the surrounding ECM was calculated using the finite element method. The viscoelastic properties of the ECM (a needed input for the finite element method solver) were obtained through oscillatory shear flow experiments on a rheometer. For validation purpose, the forces exerted by an aggregate on a force-sensor array were measured and compared against the computational results.

Life sciences require the third dimension

Novel technologies are required for three-dimensional cell biology and biophysics. By three-dimensional we refer to experimental conditions that essentially try to avoid hard and flat surfaces and favour unconstrained sample dynamics. We believe that light-sheet-based microscopes are particularly well suited to studies of sensitive three-dimensional biological systems. The application of such instruments can be illustrated with examples from the biophysics of microtubule dynamics and three-dimensional cell cultures. Our experience leads us to suggest that three-dimensional approaches reveal new aspects of a system and enable experiments to be performed in a more physiological and hence clinically more relevant context.

Alpha-catenin is a molecular switch that binds E-cadherin-beta-catenin and regulates actin-filament assembly

Epithelial cell-cell junctions, organized by adhesion proteins and the underlying actin cytoskeleton, are considered to be stable structures maintaining the structural integrity of tissues. Contrary to the idea that alpha-catenin links the adhesion protein E-cadherin through beta-catenin to the actin cytoskeleton, in the accompanying paper we report that alpha-catenin does not bind simultaneously to both E-cadherin-beta-catenin and actin filaments. Here we demonstrate that alpha-catenin exists as a monomer or a homodimer with different binding properties. Monomeric alpha-catenin binds more strongly to E-cadherin-beta-catenin, whereas the dimer preferentially binds actin filaments. Different molecular conformations are associated with these different binding states, indicating that alpha-catenin is an allosteric protein. Significantly, alpha-catenin directly regulates actin-filament organization by suppressing Arp2/3-mediated actin polymerization, likely by competing with the Arp2/3 complex for binding to actin filaments. These results indicate a new role for alpha-catenin in local regulation of actin assembly and organization at sites of cadherin-mediated cell-cell adhesion.

Tight junction biology and kidney dysfunction

The epithelial tight junction (TJ) has three major functions. As a "gate," it serves as a regulatory barrier separating and maintaining biological fluid compartments of different composition. As a "fence," it generates and maintains the apicobasal polarity of cells that form the confluent epithelium. Finally, the TJ proteins form a trafficking and signaling platform that regulates cell growth, proliferation, differentiation, and dedifferentiation. Six examples are selected that illustrate the emerging link between TJ dysfunction and kidney disease. First, the glomerular slit diaphragm (GSD) is evolved, in part, from the TJ and, on maturation, exhibits all three functions of the TJ. GSD dysfunction leads to proteinuria and, in some instances, podocyte dedifferentiation and proliferation. Second, accumulating evidence supports epithelial-mesenchymal transformation (EMT) as a major player in renal fibrosis, the final common pathway that leads to end-stage renal failure. EMT is characterized by a loss of cell-cell contact and apicobasal polarity, which are hallmarks of TJ dysfunction. Third, in autosomal dominant polycystic kidney disease, mutations of the polycystins may disrupt their known interactions with the apical junction complex, of which the TJ is a major component. This can lead to disturbances in epithelial polarity regulation with consequent abnormal tubulogenesis and cyst formation. Fourth, evidence for epithelial barrier and polarity dysregulation in the pathogenesis of ischemic acute renal failure will be summarized. Fifth, the association between mutations of paracellin-1, the first TJ channel identified, and clinical disorders of magnesium and calcium wasting and bovine renal fibrosis will be used to highlight an integral TJ protein that can serve multiple TJ functions. Finally, the role of WNK4 protein kinase in shunting chloride across the TJ of the distal nephron will be addressed.

Ligand-induced internalization selects use of common receptor neuropilin-1 by VEGF165 and semaphorin3A

Neuropilin-1 (Npn-1) is a receptor shared by class 3 semaphorins and heparin-binding forms of vascular endothelial growth factor (VEGF), protein families that regulate endothelial and neuronal-cell function. Ligand interaction with Npn-1 dictates the choice of signal transducer; plexins transduce semaphorin signals, and VEGF receptors transduce VEGF signals. It is not clear how class 3 semaphorins affect endothelial-cell function and how the shared receptor Npn-1 selects its ligand. We report that semaphorin3A (Sema3A) inhibits endothelial-cell lamellipodia formation, adhesion, survival, proliferation, and cord formation. VEGF(165), but not VEGF(121), could block all these effects of Sema3A. VEGF(165) competed with Sema3A for binding to endothelial cells, effectively reduced cell-surface Npn-1, and promoted its internalization. Use of soluble forms of Npn-1 or VEGF receptor-1 to block VEGF(165) binding to Npn-1 or to VEGF receptors provided evidence that surface Npn-1 and VEGF receptors are required for VEGF(165)-induced Npn-1 internalization. Sema3A also reduced cell-surface Npn-1 in endothelial cells and promoted its internalization, but required a higher concentration than VEGF(165). These results demonstrate that preferential receptor binding and internalization by a ligand are mechanisms by which the common receptor Npn-1 can play an essential role in prioritizing conflicting signals.

Blocked Acinar Development, E-Cadherin Reduction, and Intraepithelial Neoplasia upon Ablation of p120-Catenin in the Mouse Salivary Gland

p120 catenin is thought to be a key regulator of E-cadherin function and stability, but its role(s) in vivo is poorly understood. To examine these directly, we generated a conditional p120 knockout mouse and targeted p120 ablation to the embryonic salivary gland. Surprisingly, acinar differentiation is completely blocked, resulting in a gland composed entirely of ducts. Moreover, p120 ablation causes E-cadherin deficiency in vivo and severe defects in adhesion, cell polarity, and epithelial morphology. These changes closely phenocopy high-grade intraepithelial neoplasia, a condition that, in humans, typically progresses to invasive cancer. Tumor-like protrusions appear immediately after p120 ablation at e14 and expand into the lumen until shortly after birth, at which time the animals die with completely occluded glands. The data reveal an unexpected role for p120 in salivary acinar development and show that p120 ablation by itself induces effects consistent with a role in tumor progression.

serpentine and vermiform Encode Matrix Proteins with Chitin Binding and Deacetylation Domains that Limit Tracheal Tube Length in Drosophila

Many organs contain epithelial tubes that transport gases or liquids . Proper tube size and shape is crucial for organ function, but the mechanisms controlling tube diameter and length are poorly understood. Recent studies of tracheal (respiratory) tube morphogenesis in Drosophila show that chitin synthesis genes produce an expanding chitin cylinder in the apical (luminal) extracellular matrix (ECM) that coordinates the dilation of the surrounding epithelium . Here, we describe two genes involved in chitin modification, serpentine (serp) and vermiform (verm), mutations in which cause excessively long and tortuous tracheal tubes. The genes encode similar proteins with an LDL-receptor ligand binding motif and chitin binding and deacetylation domains. Both proteins are expressed and secreted during tube expansion and localize throughout the lumen in a chitin-dependent manner. Unlike previously characterized chitin pathway genes, serp and verm are not required for chitin synthesis or secretion but rather for its normal fibrillar structure. The mutations also affect structural properties of another chitinous matrix, epidermal cuticle. Our work demonstrates that chitin and the matrix proteins Serp and Verm limit tube elongation, and it suggests that tube length is controlled independently of diameter by modulating physical properties of the chitin ECM, presumably by N-deacetylation of chitin and conversion to chitosan.

Dlk1 expression marks developing endothelium and sites of branching morphogenesis in the mouse embryo and placenta

The protein product of the Delta-like 1 (Dlk1) gene belongs to the Delta-Notch family of signaling molecules, proteins involved in cell fate determination in many tissues during development. The DLK1 protein is believed to function as a growth factor, maintaining the proliferative state of undifferentiated cells, and is usually down-regulated as immature cells differentiate. The expression pattern of the DLK1 protein has been described in certain human tissues; however, Dlk1 expression is not well understood in the mouse, the most tractable mammalian genetic model system. To better understand the role of Dlk1 in embryonic development, the tissue-specific expression pattern of Dlk1 mRNA during mouse embryogenesis was analyzed by in situ hybridization. In embryonic day 12.5 (e12.5) embryos, high levels of Dlk1 were found in the developing pituitary, pancreas, lung, adrenal, and many mesodermally derived tissues. Strikingly, Dlk1 expression also marks the growing branches of organs that develop through the process of branching morphogenesis. At e16.5, Dlk1 expression is down-regulated in most tissues but remains in the pituitary, the adrenal gland, and in skeletal muscle. In the placenta, expression of Dlk1 is detected in endothelial cells lining the fetal blood vessels of the labyrinth. This pattern is distinct from that seen in the human placenta and suggests a role for Dlk1 in regulating maternal-fetal interactions.

Endothelial extracellular matrix: biosynthesis, remodeling, and functions during vascular morphogenesis and neovessel stabilization

The extracellular matrix (ECM) is critical for all aspects of vascular biology. In concert with supporting cells, endothelial cells (ECs) assemble a laminin-rich basement membrane matrix that provides structural and organizational stability. During the onset of angiogenesis, this basement membrane matrix is degraded by proteinases, among which membrane-type matrix metalloproteinases (MT-MMPs) are particularly significant. As angiogenesis proceeds, ECM serves essential functions in supporting key signaling events involved in regulating EC migration, invasion, proliferation, and survival. Moreover, the provisional ECM serves as a pliable scaffold wherein mechanical guidance forces are established among distal ECs, thereby providing organizational cues in the absence of cell-cell contact. Finally, through specific integrin-dependent signal transduction pathways, ECM controls the EC cytoskeleton to orchestrate the complex process of vascular morphogenesis by which proliferating ECs organize into multicellular tubes with functional lumens. Thus, the composition of ECM and therefore the regulation of ECM degradation and remodeling serves pivotally in the control of lumen and tube formation and, finally, neovessel stability and maturation.

Regulation of epithelial tubule formation by Rho family GTPases

Previous work has established that the integrin signal transduction pathway plays an important role in the regulation of epithelial tubule formation. Furthermore, it has been demonstrated that Rho-kinase, an effector of the Rho signaling pathway, is an important downstream modulator of collagen-mediated renal and mammary epithelial tubule morphogenesis. In the present study, MDCK cells that expressed mutant dominant-negative, constitutively active Rho family GTPases were used to provide further insight into Rho-GTPase signaling and the regulation of epithelial tubule formation. Using collagen gel overlays on MDCK cells as a model system, we observed phosphorylated myosin light chain (pMLC) at the leading edge of migrating lamellipodia. This epithelial remodeling led to the formation of multicellular branching epithelial tubular structures with extensive tight junctions. However, in cells expressing dominant-negative RhoN19, MLC phosphorylation, epithelial remodeling, and tubule formation were inhibited. Instead, only small apical lumens with a solitary tight junctional ring were observed, providing further evidence that Rho signaling through Rho-kinase is important in the regulation of epithelial tubule formation. Because the present model for the Rho signaling pathway proposes that Rac plays a prominent but reciprocal role in cell regulation, experiments were conducted using cells that expressed constitutively active RacV12. When incubated with collagen gels, RacV12-expressing cells formed small apical lumens with simple tight junctions, suggesting that Rac1 signaling also has a prominent role in the regulation of epithelial morphogenesis. Complementary collagen gel overlay experiments with wild-type MDCK cells demonstrated that endogenous Rac1 activation levels decreased over a time course consistent with lamellipodia and tubule formation. Under these conditions, Rac1 was initially localized to the basolateral membrane. However, after epithelial remodeling, activated Rac1 was observed primarily in lamellipodia. These studies support a model in which Rac1 and RhoA are important modulators of epithelial tubule formation.

Formation of the digestive system in zebrafish: III. Intestinal epithelium morphogenesis

Recent analysis of a novel strain of transgenic zebrafish (gutGFP) has provided a detailed description of the early morphological events that occur during the development of the liver and pancreas. In this paper, we aim to complement these studies by providing an analysis of the morphological events that shape the zebrafish intestinal epithelium. One of our goals is to provide a framework for the future characterization of zebrafish mutant phenotypes in which intestinal epithelial morphogenesis has been disrupted. Our analysis encompasses the period between 26 and 126 h post-fertilization (hpf) and follows the growth, lumen formation and differentiation of a continuous layer of endoderm into a functional intestinal epithelium with three morphologically distinct segments: the intestinal bulb, mid-intestine and posterior intestine. Between 26 hpf and 76 hpf, the entire intestinal endoderm is a highly proliferative organ. To make a lumen, the zebrafish endoderm cells undergo apical membrane biogenesis, adopt a bilayer configuration and form small cavities that coalesce without cell death. Thereafter, the endoderm cells polarize and differentiate into distinct cell lineages. Enteroendocrine cells are distinguished first at 52 hpf in the caudal region of the intestine in a new stable transgenic line, Tg[nkx2.2a:mEGFP]. The differentiation of mucin-containing goblet cells is first evident at 100 hpf and is tightly restricted to a middle segment of the intestine, designated the mid-intestine, that is also demarcated by the presence of enterocytes with large supranuclear vacuoles. Meanwhile, striking expansion of the lumen in the rostral intestine forms the intestinal bulb. Here the epithelium elaborates folds and proliferating cells become progressively restricted to a basal compartment analogous to the crypts of Lieberkühn in mammals. At 126 hpf, the posterior intestine remains an unfolded monolayer of simple columnar epithelium.

Key stages in mammary gland development: the cues that regulate ductal branching morphogenesis

Part of how the mammary gland fulfills its function of producing and delivering adequate amounts of milk is by forming an extensive tree-like network of branched ducts from a rudimentary epithelial bud. This process, termed branching morphogenesis, begins in fetal development, pauses after birth, resumes in response to estrogens at puberty, and is refined in response to cyclic ovarian stimulation once the margins of the mammary fat pad are met. Thus it is driven by systemic hormonal stimuli that elicit local paracrine interactions between the developing epithelial ducts and their adjacent embryonic mesenchyme or postnatal stroma. This local cellular cross-talk, in turn, orchestrates the tissue remodeling that ultimately produces a mature ductal tree. Although the precise mechanisms are still unclear, our understanding of branching in the mammary gland and elsewhere is rapidly improving. Moreover, many of these mechanisms are hijacked, bypassed, or corrupted during the development and progression of cancer. Thus a clearer understanding of the underlying endocrine and paracrine pathways that regulate mammary branching may shed light on how they contribute to cancer and how their ill effects might be overcome or entirely avoided.

Distinct Roles for Two Receptor Tyrosine Kinases in Epithelial Branching Morphogenesis in Drosophila

Branching morphogenesis is a widespread mechanism used to increase the surface area of epithelial organs. Many signaling systems steer development of branched organs, but it is still unclear which cellular processes are regulated by the different pathways. We have used the development of the air sacs of the dorsal thorax of Drosophila to study cellular events and their regulation via cell-cell signaling. We find that two receptor tyrosine kinases play important but distinct roles in air sac outgrowth. Fgf signaling directs cell migration at the tip of the structure, while Egf signaling is instrumental for cell division and cell survival in the growing epithelial structure. Interestingly, we find that Fgf signaling requires Ras, the Mapk pathway, and Pointed to direct migration, suggesting that both cytoskeletal and nuclear events are downstream of receptor activation. Ras and the Mapk pathway are also needed for Egf-regulated cell division/survival, but Pointed is dispensable.

Requirement for chitin biosynthesis in epithelial tube morphogenesis

Many organs are composed of branched networks of epithelial tubes that transport vital fluids or gases. The proper size and shape of tubes are crucial for their transport function, but the molecular processes that govern tube size and shape are not well understood. Here we show that three genes required for tracheal tube morphogenesis in Drosophila melanogaster encode proteins involved in the synthesis and accumulation of chitin, a polymer of N-acetyl-beta-D-glucosamine that serves as a scaffold in the rigid extracellular matrix of insect cuticle. In all three mutants, developing tracheal tubes bud and extend normally, but the epithelial walls of the tubes do not expand uniformly, and the resultant tubes are grossly misshapen, with constricted and distended regions all along their lengths. The genes are expressed in tracheal cells during the expansion process, and chitin accumulates in the lumen of tubes, forming an expanding cylinder that we propose coordinates the behavior of the surrounding tracheal cells and stabilizes the expanding epithelium. These findings show that chitin regulates epithelial tube morphogenesis, in addition to its classical role protecting mature epithelia.

A transient luminal chitinous matrix is required to model epithelial tube diameter in the Drosophila trachea

Epithelial tubes are found in many vital organs and require uniform and correct tube diameters for optimal function. Tube size depends on apical membrane growth and subapical cytoskeletal reorganization, but the cues that coordinate these events to ensure functional tube shape remain elusive. We find that epithelial tubes in the Drosophila trachea require luminal chitin polysaccharides to attain the correct diameter. Tracheal chitin forms a broad transient filament within the tubes during the restricted period of expansion. Loss of chitin causes tubular constrictions and cysts associated with irregular subapical cytoskeletal organization, without affecting epithelial integrity and polarity. Analysis of previously identified tube expansion mutants in genes encoding septate junction proteins further suggests that septate junction components may function in tubulogenesis through their role in luminal matrix assembly. We propose that the transient luminal protein/polysaccharide matrix is sensed by the epithelial cells and coordinates cytoskeletal organization to ensure uniform lumen diameter.

mummy/cystic encodes an enzyme required for chitin and glycan synthesis, involved in trachea, embryonic cuticle and CNS development--analysis of its role in Drosophila tracheal morphogenesis

Tracheal and nervous system development are two model systems for the study of organogenesis in Drosophila. In two independent screens, we identified three alleles of a gene involved in tracheal, cuticle and CNS development. Here, we show that these alleles, and the previously identified cystic and mummy, all belong to the same complementation group. These are mutants of a gene encoding the UDP-N-acetylglucosamine diphosphorylase, an enzyme responsible for the production of UDP-N-acetylglucosamine, an important intermediate in chitin and glycan biosynthesis. cyst was originally singled out as a gene required for the regulation of tracheal tube diameter. We characterized the cyst/mmy tracheal phenotype and upon histological examination concluded that mmy mutant embryos lack chitin-containing structures, such as the procuticle at the epidermis and the taenidial folds in the tracheal lumen. While most of their tracheal morphogenesis defects can be attributed to the lack of chitin, when compared to krotzkopf verkehrt (kkv) chitin-synthase mutants, mmy mutants showed a stronger phenotype, suggesting that some of the mmy phenotypes, like the axon guidance defects, are chitin-independent. We discuss the implications of these new data in the mechanism of size control in the Drosophila trachea.

Role of physical mechanisms in biological self-organization

Organs form during morphogenesis, the process that gives rise to specialized biological structures of specific shape and function in early embryonic development. Morphogenesis is under strict genetic control, but shape evolution itself is a physical process. Here we report the results of experimental and modeling biophysical studies on in vitro biological structure formation. Experimentally, by controlling the interaction between cells and their embedding matrices, we were able to build living structures of definite geometry. The experimentally observed shape evolution was reproduced by Monte Carlo simulations, which also shed light on the biophysical basis of the process. Our work suggests a novel way to engineer biological structures of controlled shape.

Molecular pathogenesis of ADPKD: the polycystin complex gets complex

Autosomal-dominant polycystic kidney disease (ADPKD) is one of the most common human monogenic diseases with an incidence of 1:400 to 1:1000. It is characterized by the progressive development and enlargement of focal cysts in both kidneys, typically resulting in end-stage renal disease (ESRD) by the fifth decade. The cystogenic process is highly complex with a cellular phenotype consistent with "dedifferentiation" (i.e., a high proliferative rate, increased apoptosis, altered protein sorting, changed secretory characteristics, and disorganization of the extracellular matrix). Although cystic renal disease is the major cause of morbidity, the occurrence of nonrenal cysts, most notably in the liver (occasionally resulting in clinically significant polycystic liver disease) and the increased prevalence of other abnormalities including intracranial aneurysms, indicate that ADPKD is a systemic disorder. Following the identification of the first ADPKD gene, PKD1, 10 years ago and PKD2 2 years later, considerable progress has been made in defining the etiology and understanding the pathogenesis of this disorder, knowledge that is now leading to the development of several promising new therapies. The purpose of this review is to summarize our current state of knowledge as to the structure and function of the PKD1 and PKD2 proteins, polycystin-1 and -2, respectively, and explore how mutation at these loci results in the spectrum of changes seen in ADPKD.

Adhesion proteins and the control of cell shape

The adherens junction functions to connect epithelial cells and maintain their polarized architecture. The geometry of the adherens junction, and consequently the shape of a cell, appears to reach an energetically favorable state. Cadherins within the adherens junction are necessary for cells to achieve this state. However, the view of an adherens junction as a static structure is at odds with the highly dynamic properties of epithelia during development. Interactions between the actin cytoskeleton and the adherens junction are required for certain cell shape changes. Recent insights into adherens junction remodeling have revealed the importance of polarized localization of myosin and Par3 at the adherens junction.

C. elegans daf-6 encodes a patched-related protein required for lumen formation

Sensory organs are often composed of neuronal sensory endings accommodated in a lumen formed by ensheathing epithelia or glia. Here we show that lumen formation in the C. elegans amphid sensory organ requires the gene daf-6. daf-6 encodes a Patched-related protein that localizes to the luminal surfaces of the amphid channel and other C. elegans tubes. While daf-6 mutants display only amphid lumen defects, animals defective for both daf-6 and the Dispatched gene che-14 exhibit defects in all tubular structures that express daf-6. Furthermore, DAF-6 protein is mislocalized, and lumen morphogenesis is abnormal, in mutants with defective sensory neuron endings. We propose that amphid lumen morphogenesis is coordinated by neuron-derived cues and a DAF-6/CHE-14 system that regulates vesicle dynamics during tubulogenesis.

Distinct and sequential tissue-specific activities of the LIM-class homeobox gene Lim1 for tubular morphogenesis during kidney development

Kidney organogenesis requires the morphogenesis of epithelial tubules. Inductive interactions between the branching ureteric buds and the metanephric mesenchyme lead to mesenchyme-to-epithelium transitions and tubular morphogenesis to form nephrons, the functional units of the kidney. The LIM-class homeobox gene Lim1 is expressed in the intermediate mesoderm, nephric duct, mesonephric tubules, ureteric bud, pretubular aggregates and their derivatives. Lim1-null mice lack kidneys because of a failure of nephric duct formation, precluding studies of the role of Lim1 at later stages of kidney development. Here, we show that Lim1 functions in distinct tissue compartments of the developing metanephros for both proper development of the ureteric buds and the patterning of renal vesicles for nephron formation. These observations suggest that Lim1 has essential roles in multiple steps of epithelial tubular morphogenesis during kidney organogenesis. We also demonstrate that the nephric duct is essential for the elongation and maintenance of the adjacent Mullerian duct, the anlage of the female reproductive tract.

Making tubes in the Drosophila embryo

Epithelial and endothelial tubes come in various shapes and sizes and form the basic units of many tubular organs. During embryonic development, single unbranched tubes as well as highly branched networks of tubes form from simple sheets of cells by several morphogenic movements. Studies of tube formation in the Drosophila embryo have greatly advanced our understanding of the cellular and molecular mechanisms by which tubes are formed. This review highlights recent progress on formation of the hindgut, Malpighian tubules, proventriculus, salivary gland, and trachea of the Drosophila embryo, focusing on the cellular events that form each tube and their genetic requirements.

Bim regulation of lumen formation in cultured mammary epithelial acini is targeted by oncogenes

Epithelial cells organize into cyst-like structures that contain a spherical monolayer of cells that enclose a central lumen. Using a three-dimensional basement membrane culture model in which mammary epithelial cells form hollow, acinus-like structures, we previously demonstrated that lumen formation is achieved, in part, through apoptosis of centrally localized cells. We demonstrate that the proapoptotic protein Bim may selectively trigger apoptosis of the centrally localized acinar cells, leading to temporally controlled lumen formation. Bim is not detectable during early stages of three-dimensional mammary acinar morphogenesis and is then highly upregulated in all cells of acini, coincident with detection of apoptosis in the centrally localized acinar cells. Inhibition of Bim expression by RNA interference transiently blocks luminal apoptosis and delays lumen formation. Oncogenes that induce acinar luminal filling, such as ErbB2 and v-Src, suppress expression of Bim through a pathway dependent on Erk-mitogen-activated protein kinase; however, HPV 16 E7, an oncogene that stimulates cell proliferation but not luminal filling, is unable to reduce Bim expression. Thus, Bim is a critical regulator of luminal apoptosis during mammary acinar morphogenesis in vitro and may be an important target of oncogenes that disrupt glandular epithelial architecture.

Juxtaposition between two cell types is necessary for dorsal appendage tube formation

The Drosophila egg chamber provides an excellent model for studying the link between patterning and morphogenesis. Late in oogenesis, a portion of the flat follicular epithelium remodels to form two tubes; secretion of eggshell proteins into the tube lumens creates the dorsal appendages. Two distinct cell types contribute to dorsal appendage formation: cells expressing the rhomboid-lacZ (rho-lacZ) marker form the ventral floor of the tube and cells expressing high levels of the transcription factor Broad form a roof over the rho-lacZ cells. In mutants that produce defective dorsal appendages (K10, Ras and ectopic decapentaplegic) both cell types are specified and reorganize to occupy their stereotypical locations within the otherwise defective tubes. Although the rho-lacZ and Broad cells rearrange to form a tube in wild type and mutant egg chambers, they never intermingle, suggesting that a boundary exists that prevents mixing between these two cell types. Consistent with this hypothesis, the Broad and rho-lacZ cells express different levels of the homophilic adhesion molecule Fasciclin 3. Furthermore, in the anterior of the egg, ectopic rhomboid is sufficient to induce both cell types, which reorganize appropriately to form an ectopic tube. We propose that signaling across a boundary separating the rho-lacZ and Broad cells choreographs the cell shape-changes and rearrangements necessary to transform an initially flat epithelium into a tube.

Three-dimensional polarization sensitizes hepatocytes to Fas/CD95 apoptotic signalling

Maintenance of epithelial cell shape and polarity determines many vital cell functions, including the appropriate response to external stimuli. Murine hepatocytes cultured in a three-dimensional Matrigel matrix formed highly polarized organoids characterized by specific localization of an ERM (ezrin/radixin/moesin) protein, radixin, at microvillus-lined membrane domains. These apical domains surrounded a lumen and were bordered by tight junctions. The hepatocyte organoids were functional as judged by the high level of albumin secretion and accumulation of bilirubin. Stimulation of the Fas/CD95 death receptor, which is highly hepatotoxic in vivo, was a strong inducer of apoptosis in the polarized organoids. This was in sharp contrast to the monolayer hepatocyte cultures, which were protected from death by exacerbated NF-κB signalling following engagement of the death receptors. Thus, hepatocytes in polarized, functional organoids modulate an intracellular signal transduction pathway, allowing the recapitulation of their physiological response to an apoptotic stimulus.

Genetic control of cell intercalation during tracheal morphogenesis in Drosophila

BACKGROUND

Branching morphogenesis transforms an epithelial sheet into a tubular network with distinct features regarding the length and diameter of individual tubes. Branching is controlled by several signaling pathways, but the molecular consequences of these pathways in the responding cells are poorly understood.

RESULTS

We have undertaken a detailed characterization of cell rearrangements during tracheal branching morphogenesis in Drosophila embryos with a GFP fusion protein labeling the adherens junctions (AJs) and high-resolution live imaging. To analyze the branching process at the cellular level, we further developed an imaging approach that allows us to follow single cells during the branching process. We find that controlled cell intercalation, which requires extensive AJ remodeling, is key to the formation of tracheal branches of different cellular complexities. In particular, most branches consist of tubes with individual cells wrapped around the lumen. These branches form through cell intercalation, which requires the transformation of most of the initial intercellular AJs into autocellular AJs. We propose a step-wise model explaining how this AJ remodeling occurs and use this model to better understand defects in various mutants. We find that Dpp and Wnt signaling control cell intercalation by regulating the expression of Spalt, a zinc finger transcription factor; Spalt inhibits intercalation, leading to the formation of large, multicellular tubes.

CONCLUSION

Tracheal morphogenesis is regulated by an interplay of different signaling systems that control cell migration and cell intercalation, respectively. Only the combined action of these signaling systems allows efficient branch elongation and the formation of morphologically distinct branches.

Molecular Identification of Canine Podocalyxin-Like Protein 1 as a Renal Tubulogenic Regulator

GP135 is an apical membrane protein expressed in polarized MDCK epithelial cells. When cultured in three-dimensional collagen gel, MDCK cells form branching tubules in response to hepatocyte growth factor stimulation in a manner that simulates the embryonic renal development. During this process, GP135 displays transient loss of membranous localization but reappears at the cell surface when nascent lumen emerges from the developing tubules. Despite being used for decades as the canonical hallmark of apical surface, the molecular identity and the significance of the dynamic expression of GP135 during the tubulogenic process remain elusive. For exploring the function of GP135, the full-length cDNA encoding GP135 was obtained. Sequence alignments and features analysis confirm GP135 as a canine homolog of podocalyxin, confirming the finding of an earlier independent study. Immunohistochemical assays on canine kidney sections identified both glomerular and tubular distribution of GP135 along the nephron. Mutant MDCK cells expressing siRNA targeted at two regions of GP135 show defects in hepatocyte growth factor-induced tubulogenesis. Re-expression of full-length and an O-linked glycosylation abbreviated construct of GP135 could recapitulate the tubulogenesis process lacking in siRNA knockdown cells; however, a deletion construct devoid of the cytoplasmic domain failed to rescue the phenotype. In summary, the data identify the MDCK apical domain marker GP135 as a tubular form of podocalyxin and provide evidence for its importance in renal tubulogenesis.

Growth of intestinal epithelium in organ culture is dependent on EGF signalling

Differentiation of endoderm into intestinal epithelium is initiated at E13.5 of mouse development when there are significant changes in morphology resulting in the conversion of undifferentiated stratified epithelium into a mature epithelial monolayer. Here we demonstrate that monolayer formation is associated with the selective apoptosis of superficial cells lining the lumen while cell proliferation is progressively restricted to cells adjacent to the basement membrane. We describe an innovative embryonic gut culture system that maintains the three-dimensional architecture of gut and in which these processes are recapitulated in vitro. Explants taken from specific regions of the gut and placed into organ culture develop and express molecular markers (Cdx1, Cdx2 and A33 antigen) in the same spatial and temporal pattern observed in vivo indicating that regional specification is maintained. Inhibition of the epidermal growth factor receptor (EGFR) tyrosine kinase using the specific inhibitor AG1478 significantly reduced the proliferation and survival of cells within the epithelial cell layer of cultured gut explants. This demonstrates an essential role for the EGF signalling pathway during the early stages of intestinal development.

A junctional problem of apical proportions: epithelial tube-size control by septate junctions in the Drosophila tracheal system

The size of epithelial tubes is critical for the function of organs such as the lung, kidney and vascular system. However, the molecular mechanisms regulating tube size are largely unknown. Recent work in the Drosophila tracheal system reveals that septate junctions play a previously unsuspected role in tube-size control. Surprisingly, this tube-size function is distinct from the established diffusion barrier function of septate junctions, and involves regulation of cell shape rather than cell number. Possible tube-size functions of septate junctions include patterning of the apical extracellular matrix and regulation of conserved cell polarity genes such as Scribble and Discs Large.

Cell-autonomous notch signaling regulates endothelial cell branching and proliferation during vascular tubulogenesis

The requirement for notch signaling during vascular development is well-documented but poorly understood. Embryonic and adult endothelial cells (EC) express notch and notch ligands; however, the necessity for cell-autonomous notch signaling during angiogenesis has not been determined. During angiogenesis, EC display plasticity, whereby a subset of previously quiescent cells loses polarity and becomes migratory. To investigate the role of notch in EC, we have used a three-dimensional in vitro system that models all of the early steps of angiogenesis. We find that newly forming sprouts are composed of specialized tip cells that guide the sprout and trunk cells that proliferate and rearrange to form intercellular lumens. Furthermore, we find that notch acts cell-autonomously to suppress EC proliferation, thereby regulating tube diameter. In addition, when notch signaling is blocked, tip cells divide, and both daughter cells take on a tip cell phenotype, resulting in increased branching through vessel bifurcation. In contrast, notch signaling is not required for re-establishment of EC polarity or for lumen formation. Thus, notch is used reiteratively and cell-autonomously by EC to regulate vessel diameter, to limit branching at the tip of sprouts, and to establish a mature, quiescent phenotype.

Pkd1 regulates immortalized proliferation of renal tubular epithelial cells through p53 induction and JNK activation

Autosomal dominant polycystic kidney disease (ADPKD) is the most common human monogenic genetic disorder and is characterized by progressive bilateral renal cysts and the development of renal insufficiency. The cystogenesis of ADPKD is believed to be a monoclonal proliferation of PKD-deficient (PKD(-/-)) renal tubular epithelial cells. To define the function of Pkd1, we generated chimeric mice by aggregation of Pkd1(-/-) ES cells and Pkd1(+/+) morulae from ROSA26 mice. As occurs in humans with ADPKD, these mice developed cysts in the kidney, liver, and pancreas. Surprisingly, the cyst epithelia of the kidney were composed of both Pkd1(-/-) and Pkd1(+/+) renal tubular epithelial cells in the early stages of cystogenesis. Pkd1(-/-) cyst epithelial cells changed in shape from cuboidal to flat and replaced Pkd1(+/+) cyst epithelial cells lost by JNK-mediated apoptosis in intermediate stages. In late-stage cysts, Pkd1(-/-) cells continued immortalized proliferation with downregulation of p53. These results provide a novel understanding of the cystogenesis of ADPKD patients. Furthermore, immortalized proliferation without induction of p53 was frequently observed in 3T3-type culture of mouse embryonic fibroblasts from Pkd1(-/-) mice. Thus, Pkd1 plays a role in preventing immortalized proliferation of renal tubular epithelial cells through the induction of p53 and activation of JNK.

Signals which build a tubule

The phenomenon of branching morphogenesis is a fundamental process critical for development of several tubular organs including lung, mammary gland, and kidney. In the case of kidney, the ureteric bud (UB) that extends out from a pre-existing epithelial tube, the Wolffian duct, gives rise to the branched collecting duct system while the surrounding metanephric mesenchyme undergoes mesenchymal-epithelial transition to form the proximal parts of the nephron. These events are mediated by several soluble factors that act in a cooperative fashion either as pro or anti tubulogenic factors. Among the growing list of such molecules are the members of the FGF, TGF-beta, and Wnt families as well as GDNF, HGF, and EGF. Cells respond to these soluble factors by initiating signaling pathways that regulate cell proliferation, cell migration and cell morphogenesis. These signaling pathways are also regulated in parallel by cell-cell and cell-matrix interactions, leading to the complex events necessary for tubule formation. Recent in-vitro and in-vivo studies have begun to shed light on the overall regulation of this phenomenon while the specific subcellular mechanisms are only beginning to be understood. This review focuses on our understanding of the morphogenic responses that regulate in-vitro tubulogenesis and how they may help us to ultimately understand this process in vivo in the kidney.

Quantitative evaluation and modeling of two-dimensional neovascular network complexity: the surface fractal dimension

Background

Modeling the complex development and growth of tumor angiogenesis using mathematics and biological data is a burgeoning area of cancer research. Architectural complexity is the main feature of every anatomical system, including organs, tissues, cells and sub-cellular entities. The vascular system is a complex network whose geometrical characteristics cannot be properly defined using the principles of Euclidean geometry, which is only capable of interpreting regular and smooth objects that are almost impossible to find in Nature. However, fractal geometry is a more powerful means of quantifying the spatial complexity of real objects.

Methods

This paper introduces the surface fractal dimension (D_s) as a numerical index of the two-dimensional (2-D) geometrical complexity of tumor vascular networks, and their behavior during computer-simulated changes in vessel density and distribution.

Results

We show that D_s significantly depends on the number of vessels and their pattern of distribution. This demonstrates that the quantitative evaluation of the 2-D geometrical complexity of tumor vascular systems can be useful not only to measure its complex architecture, but also to model its development and growth.

Conclusions

Studying the fractal properties of neovascularity induces reflections upon the real significance of the complex form of branched anatomical structures, in an attempt to define more appropriate methods of describing them quantitatively. This knowledge can be used to predict the aggressiveness of malignant tumors and design compounds that can halt the process of angiogenesis and influence tumor growth.

Polycystic liver and kidney diseases

There have been remarkable advances in research on polycystic liver and kidney diseases recently, covering cloning of new genes, refining disease classifications, and advances in understanding more about the molecular pathology of these diseases. Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary disease affecting kidneys. It affects 1/400 to 1/1000 live births and accounts for 5% of the end stage renal disease in the United States and Europe, and is caused by gene defects in the PKD1 or PKD2 genes. Compared to ADPKD, polycystic liver disease (PCLD) is a milder disease and does not lower life expectancy. Both diseases are usually adult-onset diseases. Defects in genes, which code the hepatocystin and SEC63 proteins, have just recently been found to cause PCLD. It now seems that ADPKD is caused by malfunction of the primary cilia, a cell organ sensing fluid movement, and that PCLD is a sequel from defects in protein processing. Autosomal recessive polycystic kidney disease (ARPKD) belongs to a group of congenital hepatorenal fibrocystic syndromes. All ARPKD patients have a gene defect in a gene called PKHD1, the protein product of which localizes to primary cilia. We summarize the present clinical and molecular knowledge of these diseases in this review.

Multiple rows of cells behind an epithelial wound edge extend cryptic lamellipodia to collectively drive cell-sheet movement

The mechanism by which epithelial, endothelial and other strongly cell-cell adhesive cells migrate collectively as continuous sheets is not clear, even though this process is crucial for embryonic development and tissue repair in virtually all multicellular animals. Wound closure in Madin-Darby canine kidney (MDCK) epithelial cell monolayers involves Rac GTPase-dependent migration of cells both at and behind the wound edge. We report here for the first time that cells behind the margin of wounded MDCK cell monolayers, even hundreds of microns from the edge, extend 'cryptic' lamellipodia against the substratum beneath cells in front of them, toward the wound, as determined by confocal, two-photon and transmission electron microscopy. These so-called submarginal cells nevertheless strictly maintain their more apical cell-cell contacts when they migrate as part of a coherent cell sheet, hiding their basal protrusions from conventional microscopy. The submarginal protrusions display the hallmarks of traditional lamellipodia based on morphology and dynamics. Cells behind the margin therefore actively crawl, instead of just moving passively when cells at the margin pull on them. The rate of migration is inversely proportional to the distance from the margin, and cells move co-ordinately, yet still in part autonomously, toward the wound area. We also clarify the ancillary role played by nonprotrusive contractile actin bundles that assemble in a Rho GTPase-dependent manner at the margin after wounding. In addition, some cell proliferation occurs at a delay after wounding but does not contribute to closure. Instead, it apparently serves to replace damaged cells so that intact spread cells can revert to their normal cuboidal morphology and the original cell density of the unbroken sheet can be restored.

Polarized sorting in epithelial cells: raft clustering and the biogenesis of the apical membrane

Polarized cells establish and maintain functionally distinct surface domains by an elaborate sorting process, which ensures accurate delivery of biosynthetic cargo to different parts of the plasma membrane. This is particularly evident in polarized epithelial cells, which have been used as a model system for studies of sorting mechanisms. The clustering of lipid rafts through the oligomerization of raft components could be utilized for segregating apical from basolateral cargo and for the generation of intracellular transport carriers. Besides functioning in polarized sorting in differentiated cells, raft clustering might also play an important role in the biogenesis of apical membrane domains during development.

MT1-MMP-dependent neovessel formation within the confines of the three-dimensional extracellular matrix

During angiogenesis, endothelial cells initiate a tissue-invasive program within an interstitial matrix comprised largely of type I collagen. Extracellular matrix–degradative enzymes, including the matrix metalloproteinases (MMPs) MMP-2 and MMP-9, are thought to play key roles in angiogenesis by binding to docking sites on the cell surface after activation by plasmin- and/or membrane-type (MT) 1-MMP–dependent processes. To identify proteinases critical to neovessel formation, an ex vivo model of angiogenesis has been established wherein tissue explants from gene-targeted mice are embedded within a three-dimensional, type I collagen matrix. Unexpectedly, neither MMP-2, MMP-9, their cognate cell-surface receptors (i.e., β₃ integrin and CD44), nor plasminogen are essential for collagenolytic activity, endothelial cell invasion, or neovessel formation. Instead, the membrane-anchored MMP, MT1-MMP, confers endothelial cells with the ability to express invasive and tubulogenic activity in a collagen-rich milieu, in vitro or in vivo, where it plays an indispensable role in driving neovessel formation.