Tube morphogenesis: making and shaping biological tubes

PTEN

The importance of PTEN in cellular function is underscored by the frequency of its deregulation in cancer. PTEN tumor-suppressor activity depends largely on its lipid phosphatase activity, which opposes PI3K/AKT activation. As such, PTEN regulates many cellular processes, including proliferation, survival, energy metabolism, cellular architecture, and motility. More than a decade of research has expanded our knowledge about how PTEN is controlled at the transcriptional level as well as by numerous posttranscriptional modifications that regulate its enzymatic activity, protein stability, and cellular location. Although the role of PTEN in cancers has long been appreciated, it is also emerging as an important factor in other diseases, such as diabetes and autism spectrum disorders. Our understanding of PTEN function and regulation will hopefully translate into improved prognosis and treatment for patients suffering from these ailments.

Vertex models of epithelial morphogenesis

The dynamic behavior of epithelial cell sheets plays a central role during numerous developmental processes. Genetic and imaging studies of epithelial morphogenesis in a wide range of organisms have led to increasingly detailed mechanisms of cell sheet dynamics. Computational models offer a useful means by which to investigate and test these mechanisms, and have played a key role in the study of cell-cell interactions. A variety of modeling approaches can be used to simulate the balance of forces within an epithelial sheet. Vertex models are a class of such models that consider cells as individual objects, approximated by two-dimensional polygons representing cellular interfaces, in which each vertex moves in response to forces due to growth, interfacial tension, and pressure within each cell. Vertex models are used to study cellular processes within epithelia, including cell motility, adhesion, mitosis, and delamination. This review summarizes how vertex models have been used to provide insight into developmental processes and highlights current challenges in this area, including progressing these models from two to three dimensions and developing new tools for model validation.

Cell death and morphogenesis during early mouse development: are they interconnected?

Shortly after implantation the embryonic lineage transforms from a coherent ball of cells into polarized cup shaped epithelium. Recently we elucidated a previously unknown apoptosis-independent morphogenic event that reorganizes the pluripotent lineage. Polarization cues from the surrounding basement membrane rearrange the epiblast into a polarized rosette-like structure, where subsequently a central lumen is established. Thus, we provided a new model revising the current concept of apoptosis-dependent epiblast morphogenesis. Cell death however has to be tightly regulated during embryogenesis to ensure developmental success. Here, we follow the stages of early mouse development and take a glimpse at the critical signaling and morphogenic events that determine cells destiny and reshape the embryonic lineage.

Control of the basement membrane and cell migration by ADAMTS proteinases: Lessons from C. elegans genetics

The members of the ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) family of secreted proteins, MIG-17 and GON-1, play essential roles in Caenorhabditis elegans gonadogenesis. The genetic and molecular analyses of these proteinases uncovered novel molecular interactions regulating the basement membrane (BM) during the migration of the gonadal leader cells. MIG-17, which is localized to the gonadal BM recruits or activates fibulin-1 and type IV collagen, which then recruits nidogen, thereby inducing the remodeling of the BM that is required for directional control of leader cell migration. GON-1 acts antagonistically with fibulin-1 to regulate the levels of type IV collagen accumulation in the gonadal BM, which facilitates active migration of the leader cells. The cooperative action of MIG-17 and GON-1 represents an excellent model for understanding the mechanisms of organogenesis mediated by ADAMTS proteinases.

"You Shall Not Pass"-tight junctions of the blood brain barrier

The structure and function of the barrier layers restricting the free diffusion of substances between the central nervous system (brain and spinal cord) and the systemic circulation is of great medical interest as various pathological conditions often lead to their impairment. Excessive leakage of blood-borne molecules into the parenchyma and the concomitant fluctuations in the microenvironment following a transient breakdown of the blood-brain barrier (BBB) during ischemic/hypoxic conditions or because of an autoimmune disease are detrimental to the physiological functioning of nervous tissue. On the other hand, the treatment of neurological disorders is often hampered as only minimal amounts of therapeutic agents are able to penetrate a fully functional BBB or blood cerebrospinal fluid barrier. An in-depth understanding of the molecular machinery governing the establishment and maintenance of these barriers is necessary to develop rational strategies allowing a controlled delivery of appropriate drugs to the CNS. At the basis of such tissue barriers are intimate cell-cell contacts (zonulae occludentes, tight junctions) which are present in all polarized epithelia and endothelia. By creating a paracellular diffusion constraint TJs enable the vectorial transport across cell monolayers. More recent findings indicate that functional barriers are already established during development, protecting the fetal brain. As an understanding of the biogenesis of TJs might reveal the underlying mechanisms of barrier formation during ontogenic development numerous in vitro systems have been developed to study the assembly and disassembly of TJs. In addition, monitoring the stage-specific expression of TJ-associated proteins during development has brought much insight into the “developmental tightening” of tissue barriers. Over the last two decades a detailed molecular map of transmembrane and cytoplasmic TJ-proteins has been identified. These proteins not only form a cell-cell adhesion structure, but integrate various signaling pathways, thereby directly or indirectly impacting upon processes such as cell-cell adhesion, cytoskeletal rearrangement, and transcriptional control. This review will provide a brief overview on the establishment of the BBB during embryonic development in mammals and a detailed description of the ultrastructure, biogenesis, and molecular composition of epithelial and endothelial TJs will be given.

Cdc42 and formin activity control non-muscle myosin dynamics during Drosophila heart morphogenesis

Cdc42 and the formins dDAAM and Diaphanous play pivotal roles in heart lumen formation through the spatiotemporal regulation of the actomyosin network.

During heart formation, a network of transcription factors and signaling pathways guide cardiac cell fate and differentiation, but the genetic mechanisms orchestrating heart assembly and lumen formation remain unclear. Here, we show that the small GTPase Cdc42 is essential for Drosophila melanogaster heart morphogenesis and lumen formation. Cdc42 genetically interacts with the cardiogenic transcription factor tinman; with dDAAM which belongs to the family of actin organizing formins; and with zipper, which encodes nonmuscle myosin II. Zipper is required for heart lumen formation, and its spatiotemporal activity at the prospective luminal surface is controlled by Cdc42. Heart-specific expression of activated Cdc42, or the regulatory formins dDAAM and Diaphanous caused mislocalization of Zipper and induced ectopic heart lumina, as characterized by luminal markers such as the extracellular matrix protein Slit. Placement of Slit at the lumen surface depends on Cdc42 and formin function. Thus, Cdc42 and formins play pivotal roles in heart lumen formation through the spatiotemporal regulation of the actomyosin network.

Parasympathetic innervation regulates tubulogenesis in the developing salivary gland

A fundamental question in development is how cells assemble to form a tubular network during organ formation. In glandular organs, tubulogenesis is a multistep process requiring coordinated proliferation, polarization and reorganization of epithelial cells to form a lumen, and lumen expansion. Although it is clear that epithelial cells possess an intrinsic ability to organize into polarized structures, the mechanisms coordinating morphogenetic processes during tubulogenesis are poorly understood. Here, we demonstrate that parasympathetic nerves regulate tubulogenesis in the developing salivary gland. We show that vasoactive intestinal peptide (VIP) secreted by the innervating ganglia promotes ductal growth, leads to the formation of a contiguous lumen, and facilitates lumen expansion through a cyclic AMP/protein kinase A (cAMP/PKA)-dependent pathway. Furthermore, we provide evidence that lumen expansion is independent of apoptosis and involves the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-regulated Cl(-) channel. Thus, parasympathetic innervation coordinates multiple steps in tubulogenesis during organogenesis.

Toward plasmonics-enabled spatiotemporal activity patterns in three-dimensional culture models

Spatiotemporal activity patterns of proteases such as matrix metalloproteinases and cysteine proteases in organs have the potential to provide insight into how organized structural patterns arise during tissue morphogenesis and may suggest therapeutic strategies to repair diseased tissues. Toward imaging spatiotemporal activity patterns, recently increased emphasis has been placed on imaging activity patterns in three-dimensional culture models that resemble tissues in vivo. Here, we briefly review key methods, based on fluorogenic modifications either to the extracellular matrix or to the protease-of-interest, that have allowed for qualitative imaging of activity patterns in three-dimensional culture models. We highlight emerging plasmonic methods that address significant improvements in spatial and temporal resolution and have the potential to enable quantitative measurement of spatiotemporal activity patterns with single-molecule sensitivity.

Spatially restricted hyaluronan production by Has2 drives epithelial tubulogenesis in vitro

Generation of branched tubes from an epithelial bud is a fundamental process in development. We hypothesized that induction of hyaluronan synthase (Has) and production of hyaluronan (HA) drives tubulogenesis in response to morphogenetic cytokines. Treatment of J3B1A mammary cells with transforming growth factor-β1 or renal MDCK and mCCD-N21 cells with hepatocyte growth factor induced strong and specific expression of Has2. Immunostaining revealed that HA was preferentially produced at the tips of growing tubules. Inhibition of HA production, either by 4-methylumbelliferone (4-MU) or by Has2 mRNA silencing, abrogated tubule formation. HA production by J3B1A and mCCD-N21 cells was associated with sustained activation of ERK and S6 phosphorylation. However, silencing of either CD44 or RHAMM (receptor for HA-mediated motility), the major HA receptors, by RNA interference, did not alter tubulogenesis, suggesting that this process is not receptor-mediated.

The actomyosin machinery is required for Drosophila retinal lumen formation

Multicellular tubes consist of polarized cells wrapped around a central lumen and are essential structures underlying many developmental and physiological functions. In Drosophila compound eyes, each ommatidium forms a luminal matrix, the inter-rhabdomeral space, to shape and separate the key phototransduction organelles, the rhabdomeres, for proper visual perception. In an enhancer screen to define mechanisms of retina lumen formation, we identified Actin5C as a key molecule. Our results demonstrate that the disruption of lumen formation upon the reduction of Actin5C is not linked to any discernible defect in microvillus formation, the rhabdomere terminal web (RTW), or the overall morphogenesis and basal extension of the rhabdomere. Second, the failure of proper lumen formation is not the result of previously identified processes of retinal lumen formation: Prominin localization, expansion of the apical membrane, or secretion of the luminal matrix. Rather, the phenotype observed with Actin5C is phenocopied upon the decrease of the individual components of non-muscle myosin II (MyoII) and its upstream activators. In photoreceptor cells MyoII localizes to the base of the rhabdomeres, overlapping with the actin filaments of the RTW. Consistent with the well-established roll of actomyosin-mediated cellular contraction, reduction of MyoII results in reduced distance between apical membranes as measured by a decrease in lumen diameter. Together, our results indicate the actomyosin machinery coordinates with the localization of apical membrane components and the secretion of an extracellular matrix to overcome apical membrane adhesion to initiate and expand the retinal lumen.

Biological tubes are integral units of tissues and organs such as lung, kidney, and the cardiovascular system. The fundamental design of tubes involves a central lumen wrapped by a sheet of cells. To function properly, the tubes require a precise genetic control over their creation, the diametric growth and maintenance of the lumen during development. In the fruit fly, Drosophila melanogaster, the photoreceptor cells of the eye form a tubular structure. The formation of the retinal lumen is critical for separating and positioning the light sensing organelles of each photoreceptor cell to achieve visual sensitivity. In an effort to investigate the mechanisms of Drosophila retinal lumen formation, we identified a contractile machinery that was present at the apical portion of photoreceptor cells. Our data is consistent with the idea that a contractile force contributes to the initial separation of the juxtaposed apical membranes and subsequent enlargement of the luminal space. Our work suggests that building a biological tube requires not only an extrinsic pushing force provided by the growing central lumen, but also a cell intrinsic pulling force powered by contraction of cells lining the lumen. Our findings expand and demonstrate the coordination of several molecular mechanisms to generate a tube.

Epithelial sheet folding induces lumen formation by Madin-Darby canine kidney cells in a collagen gel

Lumen formation is important for morphogenesis; however, an unanswered question is whether it involves the collective migration of epithelial cells. Here, using a collagen gel overlay culture method, we show that Madin-Darby canine kidney cells migrated collectively and formed a luminal structure in a collagen gel. Immediately after the collagen gel overlay, an epithelial sheet folded from the periphery, migrated inwardly, and formed a luminal structure. The inhibition of integrin-β1 or Rac1 activity decreased the migration rate of the peripheral cells after the sheets folded. Moreover, lumen formation was perturbed by disruption of apical-basolateral polarity induced by transforming growth factor-β1. These results indicate that cell migration and cell polarity play an important role in folding. To further explore epithelial sheet folding, we developed a computer-simulated mechanical model based on the rigidity of the extracellular matrix. It indicated a soft substrate is required for the folding movement.

Cargo sorting in the endocytic pathway: a key regulator of cell polarity and tissue dynamics

The establishment and maintenance of polarized plasma membrane domains is essential for cellular function and proper development of organisms. Epithelial cells polarize along two fundamental axes, the apicobasal and the planar, both depending on finely regulated protein trafficking mechanisms. Newly synthesized proteins destined for either surface domain are processed along the biosynthetic pathway and segregated into distinct subsets of transport carriers emanating from the trans-Golgi network or endosomes. This exocytic trafficking has been identified as essential for proper epithelial polarization. Accumulating evidence now reveals that endocytosis and endocytic recycling play an equally important role in epithelial polarization and the appropriate localization of key polarity proteins. Here, we review recent work in metazoan systems illuminating the connections between endocytosis, postendocytic trafficking, and cell polarity, both apicobasal and planar, in the formation of differentiated epithelial cells, and how these processes regulate tissue dynamics.

The novel Smad protein Expansion regulates the receptor tyrosine kinase pathway to control Drosophila tracheal tube size

Tubes with distinct shapes and sizes are critical for the proper function of many tubular organs. Here we describe a unique phenotype caused by the loss of a novel, evolutionarily-conserved, Drosophila Smad-like protein, Expansion. In expansion mutants, unicellular and intracellular tracheal branches develop bubble-like cysts with enlarged apical membranes. Cysts in unicellular tubes are enlargements of the apical lumen, whereas cysts in intracellular tubes are cytoplasmic vacuole-like compartments. The cyst phenotype in expansion mutants is similar to, but weaker than, that observed in double mutants of Drosophila type III receptor tyrosine phosphatases (RPTPs), Ptp4E and Ptp10D. Ptp4E and Ptp10D negatively regulate the receptor tyrosine kinase (RTK) pathways, especially epithelial growth factor receptor (EGFR) and fibroblast growth factor receptor/breathless (FGFR, Btl) signaling to maintain the proper size of unicellular and intracellular tubes. We show Exp genetically interacts with RTK signaling, the downstream targets of RPTPs. Cyst size and number in expansion mutants is enhanced by increased RTK signaling and suppressed by reduced RTK signaling. Genetic interaction studies strongly suggest that Exp negatively regulates RTK (EGFR, Btl) signaling to ensure proper tube sizes. Smad proteins generally function as intermediate components of the transforming growth factor-β (TGF-β, DPP) signaling pathway. However, no obvious genetic interaction between expansion and TGF-β (DPP) signaling was observed. Therefore, Expansion does not function as a typical Smad protein. The expansion phenotype demonstrates a novel role for Smad-like proteins in epithelial tube formation.

Developmental programs of lung epithelial progenitors: a balanced progenitor model

The daunting task of lung epithelium development is to transform a cluster of foregut progenitors into a three-dimensional (3D) tubular network with distinct cell types distributed at their appropriate locations. A complete understanding of lung development needs to address not only how, but also where, different cell types form. We propose that the lung epithelium forms through regulated deployment of three developmental programs: branching morphogenesis to expand progenitors and build a tree-like tubular network, airway differentiation to specify cells for the proximal conducting airways, and alveolar differentiation to specify cells for the peripheral gas exchange region. Each developmental program has its unique morphological features and molecular control mechanisms; their spatiotemporal coordination can be accounted for in a balanced progenitor model where progenitors balance between alternative developmental programs in response to spatiotemporal cues. This model integrates progenitor morphogenesis and differentiation, and provides new insights to lung immaturity in preterm birth and lung evolution. Advanced gene targeting and 3D imaging tools are needed to achieve a comprehensive understanding of lung epithelial progenitors on molecular, cellular, and morphological levels. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

Laminin promotes vascular network formation in 3D in vitro collagen scaffolds by regulating VEGF uptake

Angiogenesis is an essential neovascularisation process, which if recapitulated in 3D in vitro, will provide better understanding of endothelial cell (EC) behaviour. Various cell types and growth factors are involved, with vascular endothelial growth factor (VEGF) and its receptors VEGFR1 and VEGFR2 key components. We were able to control the aggregation pattern of ECs in 3D collagen hydrogels, by varying the matrix composition and/or having a source of cells signalling angiogenic proteins. These aggregation patterns reflect the different developmental pathways that ECs take to form different sized tubular structures. Cultures with added laminin and thus increased expression of α6 integrin showed a significant increase (p<0.05) in VEGFR2 positive ECs and increased VEGF uptake. This resulted in the end-to-end network aggregation of ECs. In cultures without laminin and therefore low α6 integrin expression, VEGFR2 levels and VEGF uptake were significantly lower (p<0.05). These ECs formed contiguous sheets, analogous to the 'wrapping' pathway in development. We have identified a key linkage between integrin expression on ECs and their uptake of VEGF, regulated by VEGFR2, resulting in different aggregation patterns in 3D.

Development and pathologies of the arterial wall

Arteries consist of an inner single layer of endothelial cells surrounded by layers of smooth muscle and an outer adventitia. The majority of vascular developmental studies focus on the construction of endothelial networks through the process of angiogenesis. Although many devastating vascular diseases involve abnormalities in components of the smooth muscle and adventitia (i.e., the vascular wall), the morphogenesis of these layers has received relatively less attention. Here, we briefly review key elements underlying endothelial layer formation and then focus on vascular wall development, specifically on smooth muscle cell origins and differentiation, patterning of the vascular wall, and the role of extracellular matrix and adventitial progenitor cells. Finally, we discuss select human diseases characterized by marked vascular wall abnormalities. We propose that continuing to apply approaches from developmental biology to the study of vascular disease will stimulate important advancements in elucidating disease mechanism and devising novel therapeutic strategies.

A review of the effects of the cell environment physicochemical nanoarchitecture on stem cell commitment

Physicochemical features of a cell nanoenvironment exert important influence on stem cell behavior and include the influence of matrix elasticity and topography on differentiation processes. The presence of growth factors such as TGF-β and BMPs on these matrices provides chemical cues and thus plays vital role in directing eventual stem cell fate. Engineering of functional biomimetic scaffolds that present programmed spatio-temporal physical and chemical signals to stem cells holds great promise in stem cell therapy. Progress in this field requires tacit understanding of the mechanistic aspects of cell-environment nanointeractions, so that they can be manipulated and exploited for the design of sophisticated next generation biomaterials. In this review, we report and discuss the evolution of these processes and pathways in the context of matrix adhesion as they might relate to stemness and stem cell differentiation. Super-resolution microscopy and single-molecule methods for in vitro nano-manipulation are helping to identify and characterize the molecules and mechanics of structural transitions within stem cells and matrices. All these advances facilitate research toward understanding of stem cell niche and consequently to developing new class of biomaterials helping the "used biomaterials" for applications in tissue engineering and regenerative medicine.

Fascin links Btl/FGFR signalling to the actin cytoskeleton during Drosophila tracheal morphogenesis

A key challenge in normal development and in disease is to elucidate the mechanisms of cell migration. Here we approach this question using the tracheal system of Drosophila as a model. Tracheal cell migration requires the Breathless/FGFR pathway; however, how the pathway induces migration remains poorly understood. We find that the Breathless pathway upregulates singed at the tip of tracheal branches, and that this regulation is functionally relevant. singed encodes Drosophila Fascin, which belongs to a conserved family of actin-bundling proteins involved in cancer progression and metastasis upon misregulation. We show that singed is required for filopodia stiffness and proper morphology of tracheal tip cells, defects that correlate with an abnormal actin organisation. We propose that singed-regulated filopodia and cell fronts are required for timely and guided branch migration and for terminal branching and branch fusion. We find that singed requirements rely on its actin-bundling activity controlled by phosphorylation, and that active Singed can promote tip cell features. Furthermore, we find that singed acts in concert with forked, another actin cross-linker. The absence of both cross-linkers further stresses the relevance of tip cell morphology and filopodia for tracheal development. In summary, our results on the one hand reveal a previously undescribed role for forked in the organisation of transient actin structures such as filopodia, and on the other hand identify singed as a new target of Breathless signal, establishing a link between guidance cues, the actin cytoskeleton and tracheal morphogenesis.

Cytokinesis defines a spatial landmark for hepatocyte polarization and apical lumen formation

By definition, all epithelial cells have apical-basal polarity, but it is unclear how epithelial polarity is acquired and how polarized cells engage in tube formation. Here, we show that hepatocyte polarization is linked to cytokinesis using the rat hepatocyte cell line Can 10. Before abscission, polarity markers are delivered to the site of cell division in a strict spatiotemporal order. Immediately after abscission, daughter cells remain attached through a unique disc-shaped structure, which becomes the site for targeted exocytosis, resulting in the formation of a primitive bile canaliculus. Subsequently, oriented cell division and asymmetric cytokinesis occur at the bile canaliculus midpoint, resulting in its equal partitioning into daughter cells. Finally, successive cycles of oriented cell division and asymmetric cytokinesis lead to the formation of a tubular bile canaliculus, which is shared by two rows of hepatocytes. These findings define a novel mechanism for cytokinesis-linked polarization and tube formation, which appears to be broadly conserved in diverse cell types.

EGFR controls IQGAP basolateral membrane localization and mitotic spindle orientation during epithelial morphogenesis

Establishing the correct orientation of the mitotic spindle is an essential step in epithelial cell division in order to ensure that epithelial tubules form correctly during organ development and regeneration. While recent findings have identified some of the molecular mechanisms that underlie spindle orientation, many aspects of this process remain poorly understood. Here, we have used the 3D-MDCK model system to demonstrate a key role for a newly identified protein complex formed by IQGAP1 and the epithelial growth factor receptor (EGFR) in controlling the orientation of the mitotic spindle. IQGAP1 is a scaffolding protein that regulates many cellular pathways, from cell-cell adhesion to microtubule organization, and its localization in the basolateral membrane ensures correct spindle orientation. Through its IQ motifs, IQGAP1 binds to EGFR, which is responsible for maintaining IQGAP1 in the basolateral membrane domain. Silencing IQGAP1, or disrupting the basolateral localization of either IQGAP1 or EGFR, results in a non-polarized distribution of NuMA, mitotic spindle misorientation and defects in single lumen formation.

The minus-end actin capping protein, UNC-94/tropomodulin, regulates development of the Caenorhabditis elegans intestine

BACKGROUND

Tropomodulins are actin-capping proteins that regulate the stability of the slow-growing, minus-ends of actin filaments. The C. elegans tropomodulin homolog, UNC-94, has sequence and functional similarity to vertebrate tropomodulins. We investigated the role of UNC-94 in C. elegans intestinal morphogenesis.

RESULTS

In the embryonic C. elegans intestine, UNC-94 localizes to the terminal web, an actin- and intermediate filament-rich structure that underlies the apical membrane. Loss of UNC-94 function results in areas of flattened intestinal lumen. In worms homozygous for the strong loss-of-function allele, unc-94(tm724), the terminal web is thinner and the amount of F-actin is reduced, pointing to a role for UNC-94 in regulating the structure of the terminal web. The non-muscle myosin, NMY-1, also localizes to the terminal web, and we present evidence that increasing actomyosin contractility by depleting the myosin phosphatase regulatory subunit, mel-11, can rescue the flattened lumen phenotype of unc-94 mutants.

CONCLUSIONS

The data support a model in which minus-end actin capping by UNC-94 promotes proper F-actin structure and contraction in the terminal web, yielding proper shape of the intestinal lumen. This establishes a new role for a tropomodulin in regulating lumen shape during tubulogenesis.

Interferon-induced transmembrane protein 1 regulates endothelial lumen formation during angiogenesis

OBJECTIVE

It is well established that angiogenesis is a complex and coordinated multistep process. However, there remains a lack of information about the genes that regulate individual stages of vessel formation. Here, we aimed to define the role of human interferon-induced transmembrane protein 1 (IFITM1) during blood vessel formation.

APPROACH AND RESULTS

We identified IFITM1 in a microarray screen for genes differentially regulated by endothelial cells (ECs) during an in vitro angiogenesis assay and found that IFITM1 expression was strongly induced as ECs sprouted and formed lumens. We showed by immunohistochemistry that human IFITM1 was expressed by stable blood vessels in multiple organs. siRNA-mediated knockdown of IFITM1 expression spared EC sprouting but completely disrupted lumen formation, in both in vitro and in an in vivo xeno-transplant model. ECs lacking IFITM1 underwent early stages of lumenogenesis (ie, intracellular vacuole formation) but failed to mature or expand lumens. Coimmunoprecipitation studies confirmed occludin as an IFITM1 binding partner in ECs, and immunocytochemistry showed a lack of occludin at endothelial tight junctions in the absence of IFITM1. Finally, time-lapse video microscopy revealed that IFITM1 is required for the formation of stable cell-cell contacts during endothelial lumen formation.

CONCLUSIONS

IFITM1 is essential for the formation of functional blood vessels and stabilizes EC-EC interactions during endothelial lumen formation by regulating tight junction assembly.

3D in vitro cell culture models of tube formation

Building the complex architecture of tubular organs is a highly dynamic process that involves cell migration, polarization, shape changes, adhesion to neighboring cells and the extracellular matrix, physicochemical characteristics of the extracellular matrix and reciprocal signaling with the mesenchyme. Understanding these processes in vivo has been challenging as they take place over extended time periods deep within the developing organism. Here, I will discuss 3D in vitro models that have been crucial to understand many of the molecular and cellular mechanisms and key concepts underlying branching morphogenesis in vivo.

Angiogenic sprouting is regulated by endothelial cell expression of Slug

The Snail family of zinc-finger transcription factors are evolutionarily conserved proteins that control processes requiring cell movement. Specifically, they regulate epithelial-to-mesenchymal transitions (EMT) where an epithelial cell severs intercellular junctions, degrades basement membrane and becomes a migratory, mesenchymal-like cell. Interestingly, Slug expression has been observed in angiogenic endothelial cells (EC) in vivo, suggesting that angiogenic sprouting may share common attributes with EMT. Here, we demonstrate that sprouting EC in vitro express both Slug and Snail, and that siRNA-mediated knockdown of either inhibits sprouting and migration in multiple in vitro angiogenesis assays. We find that expression of MT1-MMP, but not of VE-Cadherin, is regulated by Slug and that loss of sprouting as a consequence of reduced Slug expression can be reversed by lentiviral-mediated re-expression of MT1-MMP. Activity of MMP2 and MMP9 are also affected by Slug expression, likely through MT1-MMP. Importantly, we find enhanced expression of Slug in EC in human colorectal cancer samples compared with normal colon tissue, suggesting a role for Slug in pathological angiogenesis. In summary, these data implicate Slug as an important regulator of sprouting angiogenesis, particularly in pathological settings.

Self-organizing properties of mouse pluripotent cells initiate morphogenesis upon implantation

Transformation of pluripotent epiblast cells into a cup-shaped epithelium as the mouse blastocyst implants is a poorly understood and yet key developmental step. Studies of morphogenesis in embryoid bodies led to the current belief that it is programmed cell death that shapes the epiblast. However, by following embryos developing in vivo and in vitro, we demonstrate that not cell death but a previously unknown morphogenetic event transforms the amorphous epiblast into a rosette of polarized cells. This transformation requires basal membrane-stimulated integrin signaling that coordinates polarization of epiblast cells and their apical constriction, a prerequisite for lumenogenesis. We show that basal membrane function can be substituted in vitro by extracellular matrix (ECM) proteins and that ES cells can be induced to form similar polarized rosettes that initiate lumenogenesis. Together, these findings lead to a completely revised model for peri-implantation morphogenesis in which ECM triggers the self-organization of the embryo's stem cells.

Single continuous lumen formation in the zebrafish gut is mediated by smoothened-dependent tissue remodeling

The formation of a single lumen during tubulogenesis is crucial for the development and function of many organs. Although 3D cell culture models have identified molecular mechanisms controlling lumen formation in vitro, their function during vertebrate organogenesis is poorly understood. Using light sheet microscopy and genetic approaches we have investigated single lumen formation in the zebrafish gut. Here we show that during gut development multiple lumens open and enlarge to generate a distinct intermediate, which consists of two adjacent unfused lumens separated by basolateral contacts. We observed that these lumens arise independently from each other along the length of the gut and do not share a continuous apical surface. Resolution of this intermediate into a single, continuous lumen requires the remodeling of contacts between adjacent lumens and subsequent lumen fusion. We show that lumen resolution, but not lumen opening, is impaired in smoothened (smo) mutants, indicating that fluid-driven lumen enlargement and resolution are two distinct processes. Furthermore, we show that smo mutants exhibit perturbations in the Rab11 trafficking pathway and demonstrate that Rab11-mediated trafficking is necessary for single lumen formation. Thus, lumen resolution is a distinct genetically controlled process crucial for single, continuous lumen formation in the zebrafish gut.

Regulation of polarized morphogenesis by protein kinase C iota in oncogenic epithelial spheroids

Protein kinase C iota (PKCι), a serine/threonine kinase required for cell polarity, proliferation and migration, is commonly up- or downregulated in cancer. PKCι is a human oncogene but whether this is related to its role in cell polarity and what repertoire of oncogenes acts in concert with PKCι is not known. We developed a panel of candidate oncogene expressing Madin-Darby canine kidney (MDCK) cells and demonstrated that H-Ras, ErbB2 and phosphatidylinositol 3-kinase transformation led to non-polar spheroid morphogenesis (dysplasia), whereas MDCK spheroids expressing c-Raf or v-Src were largely polarized. We show that small interfering RNA (siRNA)-targeting PKCι decreased the size of all spheroids tested and partially reversed the aberrant polarity phenotype in H-Ras and ErbB2 spheroids only. This indicates distinct requirements for PKCι and moreover that different thresholds of PKCι activity are required for these phenotypes. By manipulating PKCι function using mutant constructs, siRNA depletion or chemical inhibition, we have demonstrated that PKCι is required for polarization of parental MDCK epithelial cysts in a 3D matrix and that there is a threshold of PKCι activity above and below which, disorganized epithelial morphogenesis results. Furthermore, treatment with a novel PKCι inhibitor, CRT0066854, was able to restore polarized morphogenesis in the dysplastic H-Ras spheroids. These results show that tightly regulated PKCι is required for normal-polarized morphogenesis in mammalian cells and that H-Ras and ErbB2 cooperate with PKCι for loss of polarization and dysplasia. The identification of a PKCι inhibitor that can restore polarized morphogenesis has implications for the treatment of Ras and ErbB2 driven malignancies.

Synergy of cell-cell repulsion and vacuolation in a computational model of lumen formation

A key step in blood vessel development (angiogenesis) is lumen formation: the hollowing of vessels for blood perfusion. Two alternative lumen formation mechanisms are suggested to function in different types of blood vessels. The vacuolation mechanism is suggested for lumen formation in small vessels by coalescence of intracellular vacuoles, a view that was extended to extracellular lumen formation by exocytosis of vacuoles. The cell–cell repulsion mechanism is suggested to initiate extracellular lumen formation in large vessels by active repulsion of adjacent cells, and active cell shape changes extend the lumen. We used an agent-based computer model, based on the cellular Potts model, to compare and study both mechanisms separately and combined. An extensive sensitivity analysis shows that each of the mechanisms on its own can produce lumens in a narrow region of parameter space. However, combining both mechanisms makes lumen formation much more robust to the values of the parameters, suggesting that the mechanisms may work synergistically and operate in parallel, rather than in different vessel types.

Endosomes derived from clathrin-independent endocytosis serve as precursors for endothelial lumen formation

Clathrin-independent endocytosis (CIE) is a form of bulk plasma membrane (PM) endocytosis that allows cells to sample and evaluate PM composition. Once in endosomes, the internalized proteins and lipids can be recycled back to the PM or delivered to lysosomes for degradation. Endosomes arising from CIE contain lipid and signaling molecules suggesting that they might be involved in important biological processes. During vasculogenesis, new blood vessels are formed from precursor cells in a process involving internalization and accumulation of endocytic vesicles. Here, we found that CIE has a role in endothelial lumen formation. Specifically, we found that human vascular endothelial cells (HUVECs) utilize CIE for internalization of distinct cargo molecules and that in three-dimensional cultures CIE membranes are delivered to the newly formed lumen.

Branching patterns emerge in a mathematical model of the dynamics of lung development

Recent experimental work has described an elegant pattern of branching in the development of the lung. Multiple forms of branching have been identified, including side branching and tip bifurcation. A particularly interesting feature is the phenomenon of 'orthogonal rotation of the branching plane'. The lung must fill 3D space with the essentially 2D phenomenon of branching. It accomplishes this by rotating the branching plane by 90° with each generation. The mechanisms underlying this rotation are not understood. In general, the programmes that underlie branching have been hypothetically attributed to genetic 'subroutines' under the control of a 'global master routine' to invoke particular subroutines at the proper time and location, but the mechanisms of these routines are not known. Here, we demonstrate that fundamental mechanisms, the reaction and diffusion of biochemical morphogens, can create these patterns. We used a partial differential equation model that postulates three morphogens, which we identify with specific molecules in lung development. We found that cascades of branching events, including side branching, tip splitting and orthogonal rotation of the branching plane, all emerge immediately from the model, without further assumptions. In addition, we found that one branching mode can be easily switched to another, by increasing or decreasing the values of key parameters. This shows how a 'global master routine' could work by the alteration of a single parameter. Being able to simulate cascades of branching events is necessary to understand the critical features of branching, such as orthogonal rotation of the branching plane between successive generations, and branching mode switch during lung development. Thus, our model provides a paradigm for how genes could possibly act to produce these spatial structures. Our low-dimensional model gives a qualitative understanding of how generic physiological mechanisms can produce branching phenomena, and how the system can switch from one branching pattern to another using low-dimensional 'control knobs'. The model provides a number of testable predictions, some of which have already been observed (though not explained) in experimental work.

Keratin 79 identifies a novel population of migratory epithelial cells that initiates hair canal morphogenesis and regeneration

The formation of epithelial tubes underlies the development of diverse organs. In the skin, hair follicles resemble tube-like structures with lumens that are generated through poorly understood cellular rearrangements. Here, we show that creation of the hair follicle lumen is mediated by early outward movement of keratinocytes from within the cores of developing hair buds. These migratory keratinocytes express keratin 79 (K79) and stream out of the hair germ and into the epidermis prior to lumen formation in the embryo. Remarkably, this process is recapitulated during hair regeneration in the adult mouse, when K79(+) cells migrate out of the reactivated secondary hair germ prior to formation of a new hair canal. During homeostasis, K79(+) cells line the hair follicle infundibulum, a domain we show to be multilayered, biochemically distinct and maintained by Lrig1(+) stem cell-derived progeny. Upward movement of these cells sustains the infundibulum, while perturbation of this domain during acne progression is often accompanied by loss of K79. Our findings uncover previously unappreciated long-distance cell movements throughout the life cycle of the hair follicle, and suggest a novel mechanism by which the follicle generates its hollow core through outward cell migration.

Functional pathways altered after silencing Pnpla6 (the codifying gene of neuropathy target esterase) in mouse embryonic stem cells under differentiation

Neuropathy target esterase (NTE) is involved in several disorders in adult organisms and embryos. A relationship between NTE and nervous system integrity and maintenance in adult systems has been suggested. NTE-related motor neuron disease is associated with the expression of a mutant form of NTE and the inhibition and further modification of NTE by organophosphorus compounds is the trigger of a delayed neurodegenerative neuropathy. Homozygotic NTE knockout mice embryos are not viable, while heterozygotic NTE knockout mice embryos yields mice with neurological disorders, which suggest that this protein plays a critical role in embryonic development. The present study used D3 mouse embryonic stem cells with the aim of gaining mechanistic insights on the role of Pnpla6 (NTE gene encoding) in the developmental process. D3 cells were silenced by lipofectamine transfection with a specific interference RNA for Pnpla6. Silencing Pnpla6 in D3 monolayer cultures reduced NTE enzymatic activity to 50% 20 h post-treatment, while the maximum loss of Pnpla6 expression reached 80% 48 h postsilencing. Pnpla6 was silenced in embryoid bodies and 545 genes were differentially expressed regarding the control 96 h after silencing, which revealed alterations in multiple genetic pathways, such as cell motion and cell migration, vesicle regulation, and cell adhesion. These findings also allow considering that these altered pathways would impair the formation of respiratory, neural, and vascular tubes causing the deficiencies observed in the in vivo development of nervous and vascular systems. Our findings, therefore, support the previous observations made in vivo concerning lack of viability of mice embryos not expressing NTE and help to understand the biology of several neurological and developmental disorders in which NTE is involved.

Regional fibronectin and collagen fibril co-assembly directs cell proliferation and microtissue morphology

The extracellular matrix protein, fibronectin stimulates cells to self-assemble into three-dimensional multicellular structures by a mechanism that requires the cell-dependent conversion of soluble fibronectin molecules into insoluble fibrils. Fibronectin also binds to collagen type I and mediates the co-assembly of collagen fibrils into the extracellular matrix. Here, the role of collagen-fibronectin binding in fibronectin-induced cellular self-assembly was investigated using fibronectin-null fibroblasts in an in vitro model of tissue formation. High resolution, two-photon immunofluorescence microscopy was combined with second harmonic generation imaging to examine spatial and temporal relationships among fibronectin and collagen fibrils, actin organization, cell proliferation, and microtissue morphology. Time course studies coupled with simultaneous 4-channel multiphoton imaging identified regional differences in fibronectin fibril conformation, collagen fibril remodeling, actin organization, and cell proliferation during three-dimensional cellular self-assembly. Regional differences in cell proliferation and fibronectin structure were dependent on both soluble fibronectin concentration and fibronectin-collagen interactions. Fibronectin-collagen binding was not necessary for either fibronectin matrix formation or intercellular cohesion. However, inhibiting fibronectin binding to collagen reduced collagen fibril remodeling, decreased fibronectin fibril extension, blocked fibronectin-induced cell proliferation, and altered microtissue morphology. Furthermore, continual fibronectin-collagen binding was necessary to maintain both cell proliferation and microtissue morphology. Collectively, these data suggest that the complex changes in extracellular matrix and cytoskeletal remodeling that mediate tissue assembly are driven, in part, by regional variations in cell-mediated fibronectin-collagen co-assembly.

On the morphogenesis of glial compartments in the sensory organs of Caenorhabditis elegans

Glial cells surround neuronal endings and isolate them within specialized compartments. This architecture is found at synapses in the central nervous system, as well as at receptive endings of sensory neurons. Recent studies are beginning to uncover the contributions of glial compartments to the functions of the ensheathed neurons. However, the cellular and molecular processes that guide compartment morphogenesis remain unknown. The main sensory organ of Caenorhabditis elegans, the amphid, provides an experimentally tractable setting in which to address the mechanisms underlying glial compartment formation. Amphid development is stereotyped and amphid structure is easily assayed. We recently uncovered a molecular tug of war that regulates the size of the amphid sensory compartment. The Nemo-like kinase LIT-1 interacts with the glial cytoskeleton to promote compartment growth, a process that also involves components of the retromer complex, while the Patched-related transmembrane protein DAF-6 keeps this expansion in check. Here we discuss how regulation of secretion by the cytoskeleton could guide the sculpting of glial compartments.

Cell interactions and patterned intercalations shape and link epithelial tubes in C. elegans

Many animal organs are composed largely or entirely of polarized epithelial tubes, and the formation of complex organ systems, such as the digestive or vascular systems, requires that separate tubes link with a common polarity. The Caenorhabditis elegans digestive tract consists primarily of three interconnected tubes—the pharynx, valve, and intestine—and provides a simple model for understanding the cellular and molecular mechanisms used to form and connect epithelial tubes. Here, we use live imaging and 3D reconstructions of developing cells to examine tube formation. The three tubes develop from a pharynx/valve primordium and a separate intestine primordium. Cells in the pharynx/valve primordium polarize and become wedge-shaped, transforming the primordium into a cylindrical cyst centered on the future lumenal axis. For continuity of the digestive tract, valve cells must have the same, radial axis of apicobasal polarity as adjacent intestinal cells. We show that intestinal cells contribute to valve cell polarity by restricting the distribution of a polarizing cue, laminin. After developing apicobasal polarity, many pharyngeal and valve cells appear to explore their neighborhoods through lateral, actin-rich lamellipodia. For a subset of cells, these lamellipodia precede more extensive intercalations that create the valve. Formation of the valve tube begins when two valve cells become embedded at the left-right boundary of the intestinal primordium. Other valve cells organize symmetrically around these two cells, and wrap partially or completely around the orthogonal, lumenal axis, thus extruding a small valve tube from the larger cyst. We show that the transcription factors DIE-1 and EGL-43/EVI1 regulate cell intercalations and cell fates during valve formation, and that the Notch pathway is required to establish the proper boundary between the pharyngeal and valve tubes.

Tubes composed of epithelial cells are universal building blocks of animal organs, and complex organs typically contain multiple interconnected tubes, such as in the digestive tract or vascular system. The nematode Caenorhabditis elegans provides a simple genetic system to study how tubes form and link. Understanding these events provides insight into basic biology, and can inform engineering strategies for building or repairing cellular tubes. A small tube called the valve connects the two major tubular organs of the nematode digestive tract, the pharynx and intestine. The pharynx and valve form from the same primordium, while the intestine forms from a separate primordium. Cells in each primordium polarize around a central axis, and valve formation involves connecting these axes. Using live imaging, we show that valve cells initially resemble other pharyngeal cells, but undergo additional and extensive intercalations around the lumenal axis, effectively squeezing a small tube from the larger primordium. Valve cells develop the same polarity axis as intestinal cells, and we show that this depends on interactions with the intestinal cells. We show that valve formation involves dynamic changes in the localization of adhesive proteins, and identify transcription factors that play a role in valve cell specification and intercalation.

Tubulogenesis

Metazoans require epithelial and endothelial tubes to transport liquids and gasses throughout their bodies. Although biological tubes may look relatively similar at first glance, there are multiple and distinct mechanisms by which tubes form and even more regulatory events driving the cell shape changes that produce tubes of specific dimensions. An overview of the current understanding of the molecular processes and physical forces involved in tubulogenesis is presented in this review and the accompanying poster.

Anion translocation through an Slc26 transporter mediates lumen expansion during tubulogenesis

Lumen formation is a critical event in biological tube formation, yet its molecular mechanisms remain poorly understood. Specifically, how lumen expansion is coordinated with other processes of tubulogenesis is not well known, and the role of membrane transporters in tubulogenesis during development has not been adequately addressed. Here we identify a solute carrier 26 (Slc26) family protein as an essential regulator of tubulogenesis using the notochord of the invertebrate chordate Ciona intestinalis as a model. Ci-Slc26aα is indispensable for lumen formation and expansion, but not for apical/luminal membrane formation and lumen connection. Ci-Slc26aα acts as an anion transporter, mediating the electrogenic exchange of sulfate or oxalate for chloride or bicarbonate and electroneutral chloride:bicarbonate exchange. Mutant rescue assays show that this transport activity is essential for Ci-Slc26aα's in vivo function. Our work reveals the consequences and relationships of several key processes in lumen formation, and establishes an in vivo assay for studying the molecular basis of the transport properties of SLC26 family transporters and their related diseases.

Focal defects in single-celled tubes mutant for Cerebral cavernous malformation 3, GCKIII, or NSF2

Tubes of differing cellular architecture connect into networks. In the Drosophila tracheal system, two tube types connect within single cells (terminal cells); however, the genes that mediate this interconnection are unknown. Here we characterize two genes that are essential for this process: lotus, required for maintaining a connection between the tubes, and wheezy, required to prevent local tube dilation. We find that lotus encodes N-ethylmaleimide sensitive factor 2 (NSF2), whereas wheezy encodes Germinal center kinase III (GCKIII). GCKIIIs are effectors of Cerebral cavernous malformation 3 (CCM3), a protein mutated in vascular disease. Depletion of Ccm3 by RNA interference phenocopies wheezy; thus, CCM3 and GCKIII, which prevent capillary dilation in humans, prevent tube dilation in Drosophila trachea. Ectopic junctional and apical proteins are present in wheezy terminal cells, and we show that tube dilation is suppressed by reduction of NSF2, of the apical determinant Crumbs, or of septate junction protein Varicose.

Modeling of the migration of endothelial cells on bioactive micropatterned polymers

In this paper, a macroscopic model describing endothelial cell migration on bioactive micropatterned polymers is presented. It is based on a system of partial differential equations of Patlak-Keller-Segel type that describes the evolution of the cell densities. The model is studied mathematically and numerically. We prove existence and uniqueness results of the solution to the differential system. We also show that fundamental physical properties such as mass conservation, positivity and boundedness of the solution are satisfied. The numerical study allows us to show that the modeling results are in good agreement with the experiments.

CTNNB1 in mesenchyme regulates epithelial cell differentiation during Müllerian duct and postnatal uterine development

Müllerian duct differentiation and development into the female reproductive tract is essential for fertility, but mechanisms regulating these processes are poorly understood. WNT signaling is critical for proper development of the female reproductive tract as evident by the phenotypes of Wnt4, Wnt5a, Wnt7a, and β-catenin (Ctnnb1) mutant mice. Here we extend these findings by determining the effects of constitutive CTNNB1 activation within the mesenchyme of the developing Müllerian duct and its differentiated derivatives. This was accomplished by crossing Amhr2-Cre knock-in mice with Ctnnb1 exon (ex) 3(f/f) mice. Amhr2-Cre(Δ/+); Ctnnb1 ex3(f/+) females did not form an oviduct, had smaller uteri, endometrial gland defects, and were infertile. At the cellular level, stabilization of CTNNB1 in the mesenchyme caused alterations within the epithelium, including less proliferation, delayed uterine gland formation, and induction of an epithelial-mesenchymal transition (EMT) event. This EMT event is observed before birth and is complete within 5 days after birth. Misexpression of estrogen receptor α in the epithelia correlated with the EMT before birth, but not after. These studies indicate that regulated CTNNB1 in mesenchyme is important for epithelial cell differentiation during female reproductive tract development.

Rasip1 regulates vertebrate vascular endothelial junction stability through Epac1-Rap1 signaling

Establishment and stabilization of endothelial tubes with patent lumens is vital during vertebrate development. Ras-interacting protein 1 (RASIP1) has been described as an essential regulator of de novo lumenogenesis through modulation of endothelial cell (EC) adhesion to the extracellular matrix (ECM). Here, we show that in mouse and zebrafish embryos, Rasip1-deficient vessels transition from an angioblast cord to a hollow tube, permit circulation of primitive erythrocytes, but ultimately collapse, leading to hemorrhage and embryonic lethality. Knockdown of RASIP1 does not alter EC-ECM adhesion, but causes cell-cell detachment and increases permeability of EC monolayers in vitro. We also found that endogenous RASIP1 in ECs binds Ras-related protein 1 (RAP1), but not Ras homolog gene family member A or cell division control protein 42 homolog. Using an exchange protein directly activated by cyclic adenosine monophosphate 1 (EPAC1)-RAP1-dependent model of nascent junction formation, we demonstrate that a fraction of the RASIP1 protein pool localizes to cell-cell contacts. Loss of RASIP1 phenocopies loss of RAP1 or EPAC1 in ECs by altering junctional actin organization, localization of the actin-bundling protein nonmuscle myosin heavy chain IIB, and junction remodeling. Our data show that RASIP1 regulates the integrity of newly formed blood vessels as an effector of EPAC1-RAP1 signaling.

The zebrafish common cardinal veins develop by a novel mechanism: lumen ensheathment

The formation and lumenization of blood vessels has been studied in some detail, but there is little understanding of the morphogenetic mechanisms by which endothelial cells (ECs) forming large caliber vessels aggregate, align themselves and finally form a lumen that can support blood flow. Here, we focus on the development of the zebrafish common cardinal veins (CCVs), which collect all the blood from the embryo and transport it back to the heart. We show that the angioblasts that eventually form the definitive CCVs become specified as a separate population distinct from the angioblasts that form the lateral dorsal aortae. The subsequent development of the CCVs represents a novel mechanism of vessel formation, during which the ECs delaminate and align along the inner surface of an existing luminal space. Thereby, the CCVs are initially established as open-ended endothelial tubes, which extend as single EC sheets along the flow routes of the circulating blood and eventually enclose the entire lumen in a process that we term ‘lumen ensheathment’. Furthermore, we found that the initial delamination of the ECs as well as the directional migration within the EC sheet depend on Cadherin 5 function. By contrast, EC proliferation within the growing CCV is controlled by Vascular endothelial growth factor C, which is provided by circulating erythrocytes. Our findings not only identify a novel mechanism of vascular lumen formation, but also suggest a new form of developmental crosstalk between hematopoietic and endothelial cell lineages.

Rab9 and retromer regulate retrograde trafficking of luminal protein required for epithelial tube length control

Apical extracellular matrix filling the lumen controls the morphology and geometry of epithelial tubes during development, yet the regulation of luminal protein composition and its role in tube morphogenesis are not well understood. Here we show that an endosomal-retrieval machinery consisting of Rab9, retromer and actin nucleator WASH (Wiskott–Aldrich Syndrome Protein and SCAR Homolog) regulates selective recycling of the luminal protein Serpentine in the Drosophila trachea. Secreted Serpentine is endocytosed and sorted into the late endosome. Vps35, WASH and actin filaments differentially localize at the Rab9-enriched subdomains of the endosomal membrane, where Serpentine containing vesicles bud off. In Rab9, Vps35 and WASH mutants, Serpentine was secreted normally into the tracheal lumen, but the luminal quantities were depleted at later stages, resulting in excessively elongated tubes. In contrast, secretion of many luminal proteins was unaffected, suggesting that retrograde trafficking of a specific class of luminal proteins is a pivotal rate-limiting mechanism for continuous tube length regulation.

The development of biological tubes is regulated by mutual interactions between cells and luminal extracellular matrix. Dong et al. show that retrograde recycling of luminal chitin deacetylase regulates Drosophila tracheal tubule geometry by restricting length independently of diameter.

Vascular development in the zebrafish

The zebrafish has emerged as an excellent vertebrate model system for studying blood and lymphatic vascular development. The small size, external and rapid development, and optical transparency of zebrafish embryos are some of the advantages the zebrafish model system offers. Multiple well-established techniques have been developed for imaging and functionally manipulating vascular tissues in zebrafish embryos, expanding on and amplifying these basic advantages and accelerating use of this model system for studying vascular development. In the past decade, studies performed using zebrafish as a model system have provided many novel insights into vascular development. In this article we discuss the amenability of this model system for studying blood vessel development and review contributions made by this system to our understanding of vascular development.

Tips, stalks, tubes: notch-mediated cell fate determination and mechanisms of tubulogenesis during angiogenesis

Angiogenesis is the process of developing vascular sprouts from existing blood vessels. Luminal endothelial cells convert into "tip" cells that contribute to the development of a multicellular stalk, which then undergoes lumen formation. In this review, we consider a variety of cellular and molecular pathways that mediate these transitions. We focus first on Notch signaling in cell fate determination as a mechanism to define tip and stalk cells. We next discuss the current models of lumen formation and describe new players in this process, such as chloride intracellular channel proteins. Finally, we consider the possible medical therapeutic benefits of understanding these processes and acknowledge potential obstacles in drug development.

Tubulogenesis in a simple cell cord requires the formation of bi-apical cells through two discrete Par domains

Apico-basal polarization is a crucial step in the de novo formation of biological tubes. In Ciona notochord, tubulogenesis occurs in a single file of cells in the absence of cell proliferation. This configuration presents a unique challenge for the formation of a continuous lumen. Here, we show that this geometric configuration is associated with a novel polarization strategy: the generation of bipolar epithelial cells possessing two apical/luminal domains instead of one, as in the conventional epithelium. At the molecular level, cells establish two discrete Par3/Par6/aPKC patches, and form two sets of tight junctions, in opposite points of the cells. The key molecule controlling the formation of both domains is Par3. Changing the position of the cells within the organ fundamentally changes their polarity and the number of apical domains they develop. These results reveal a new mechanism for tubulogenesis from the simplest cell arrangement, which occurs in other developmental contexts, including vertebrate vascular anastomosis.