From fate to function: the Drosophila trachea and salivary gland as models for tubulogenesis

Venom gland organogenesis in the common house spider

Venom is a remarkable innovation found across the animal kingdom, yet the evolutionary origins of venom systems in various groups, including spiders, remain enigmatic. Here, we investigated the organogenesis of the venom apparatus in the common house spider, Parasteatoda tepidariorum. The venom apparatus consists of a pair of secretory glands, each connected to an opening at the fang tip by a duct that runs through the chelicerae. We performed bulk RNA-seq to identify venom gland-specific markers and assayed their expression using RNA in situ hybridisation experiments on whole-mount time-series. These revealed that the gland primordium emerges during embryonic stage 13 at the chelicera tip, progresses proximally by the end of embryonic development and extends into the prosoma post-eclosion. The initiation of expression of an important toxin component in late postembryos marks the activation of venom-secreting cells. Our selected markers also exhibited distinct expression patterns in adult venom glands: sage and the toxin marker were expressed in the secretory epithelium, forkhead and sum-1 in the surrounding muscle layer, while Distal-less was predominantly expressed at the gland extremities. Our study provides the first comprehensive analysis of venom gland morphogenesis in spiders, offering key insights into their evolution and development.

A single-cell atlas of Drosophila trachea reveals glycosylation-mediated Notch signaling in cell fate specification

The Drosophila tracheal system is a favorable model for investigating the program of tubular morphogenesis. This system is established in the embryo by post-mitotic cells, but also undergoes remodeling by adult stem cells. Here, we provide a comprehensive cell atlas of Drosophila trachea using the single-cell RNA-sequencing (scRNA-seq) technique. The atlas documents transcriptional profiles of tracheoblasts within the Drosophila airway, delineating 9 major subtypes. Further evidence gained from in silico as well as genetic investigations highlight a set of transcription factors characterized by their capacity to switch cell fate. Notably, the transcription factors Pebbled, Blistered, Knirps, Spalt and Cut are influenced by Notch signaling and determine tracheal cell identity. Moreover, Notch signaling orchestrates transcriptional activities essential for tracheoblast differentiation and responds to protein glycosylation that is induced by high sugar diet. Therefore, our study yields a single-cell transcriptomic atlas of tracheal development and regeneration, and suggests a glycosylation-responsive Notch signaling in cell fate determination.

The transcription factor CREB3-2 regulated neutral lipase gene expression in ovary of Nilaparvata lugens

The cyclic AMP-responsive element-binding protein 3 (CREB3) members have unique regulatory roles in cellular lipid metabolism as transcription factors. Two CREB3 proteins in Nilaparvata lugens were identified and analyzed. In ovary, when silencing NlCREB3-2, triacylglycerol (TAG) content dramatically increased but glycerol and free fatty acid (FFA) significantly decreased, which implicated that NlCREB3-2 was involved in the lipase-related TAG metabolism. In N. lugens, five neutral lipases with complete features for TAG hydrolytic activity and high expression in ovary were focused. Among them, the expression levels of three neutral lipase genes were significantly down-regulated by NlCREB3-2 RNAi. The direct regulation of NlCREB3-2 towards the three neutral lipase genes was evidenced by the dual-luciferase reporter assay. After jointly silencing three neutral lipase genes, TAG and glycerol contents displayed similar changes as NlCREB3-2 RNAi. The study proved that NlCREB3-2 participated in TAG metabolism in ovary via the direct activation towards the ovary-specific neutral lipase genes.

The Arf-GEF GBF1 undergoes multi-domain structural shifts to activate Arf at the Golgi

Golgi homeostasis require the activation of Arf GTPases by the guanine-nucleotide exchange factor requires GBF1, whose recruitment to the Golgi represents a rate limiting step in the process. GBF1 contains a conserved, catalytic, Sec7 domain (Sec7d) and five additional (DCB, HUS, HDS1-3) domains. Herein, we identify the HDS3 domain as essential for GBF1 membrane association in mammalian cells and document the critical role of HDS3 during the development of Drosophila melanogaster. We show that upon binding to Golgi membranes, GBF1 undergoes conformational changes in regions bracketing the catalytic Sec7d. We illuminate GBF1 interdomain arrangements by negative staining electron microscopy of full-length human GBF1 to show that GBF1 forms an anti-parallel dimer held together by the paired central DCB-HUS core, with two sets of HDS1-3 arms extending outward in opposite directions. The catalytic Sec7d protrudes from the central core as a largely independent domain, but is closely opposed to a previously unassigned α-helix from the HDS1 domain. Based on our data, we propose models of GBF1 engagement on the membrane to provide a paradigm for understanding GBF1-mediated Arf activation required for cellular and organismal function.

Molecular mechanisms of tubulogenesis revealed in the sea star hydro-vascular organ

A fundamental goal in the organogenesis field is to understand how cells organize into tubular shapes. Toward this aim, we have established the hydro-vascular organ in the sea star Patiria miniata as a model for tubulogenesis. In this animal, bilateral tubes grow out from the tip of the developing gut, and precisely extend to specific sites in the larva. This growth involves cell migration coupled with mitosis in distinct zones. Cell proliferation requires FGF signaling, whereas the three-dimensional orientation of the organ depends on Wnt signaling. Specification and maintenance of tube cell fate requires Delta/Notch signaling. Moreover, we identify target genes of the FGF pathway that contribute to tube morphology, revealing molecular mechanisms for tube outgrowth. Finally, we report that FGF activates the Six1/2 transcription factor, which serves as an evolutionarily ancient regulator of branching morphogenesis. This study uncovers distinct mechanisms of tubulogenesis in vivo and we propose that cellular dynamics in the sea star hydro-vascular organ represents a key comparison for understanding the evolution of vertebrate organs.

Apocrine secretion in the salivary glands of Drosophilidae and other dipterans is evolutionarily conserved

Apocrine secretion is a transport and secretory mechanism that remains only partially characterized, even though it is evolutionarily conserved among all metazoans, including humans. The excellent genetic model organism Drosophila melanogaster holds promise for elucidating the molecular mechanisms regulating this fundamental metazoan process. Two prerequisites for such investigations are to clearly define an experimental system to investigate apocrine secretion and to understand the evolutionarily and functional contexts in which apocrine secretion arose in that system. To this end, we recently demonstrated that, in D. melanogaster, the prepupal salivary glands utilize apocrine secretion prior to pupation to deliver innate immune and defense components to the exuvial fluid that lies between the metamorphosing pupae and its chitinous case. This finding provided a unique opportunity to appraise how this novel non-canonical and non-vesicular transport and secretory mechanism is employed in different developmental and evolutionary contexts. Here we demonstrate that this apocrine secretion, which is mechanistically and temporarily separated from the exocytotic mechanism used to produce the massive salivary glue secretion (Sgs), is shared across Drosophilidae and two unrelated dipteran species. Screening more than 30 species of Drosophila from divergent habitats across the globe revealed that apocrine secretion is a widespread and evolutionarily conserved cellular mechanism used to produce exuvial fluid. Species with longer larval and prepupal development than D. melanogaster activate apocrine secretion later, while smaller and more rapidly developing species activate it earlier. In some species, apocrine secretion occurs after the secretory material is first concentrated in cytoplasmic structures of unknown origin that we name "collectors." Strikingly, in contrast to the widespread use of apocrine secretion to provide exuvial fluid, not all species use exocytosis to produce the viscid salivary glue secretion that is seen in D. melanogaster. Thus, apocrine secretion is the conserved mechanism used to realize the major function of the salivary gland in fruitflies and related species: it produces the pupal exuvial fluid that provides an active defense against microbial invasion during pupal metamorphosis.

Forced into shape: Mechanical forces in Drosophila development and homeostasis

Mechanical forces play a central role in shaping tissues during development and maintaining epithelial integrity in homeostasis. In this review, we discuss the roles of mechanical forces in Drosophila development and homeostasis, starting from the interplay of mechanics with cell growth and division. We then discuss several examples of morphogenetic processes where complex 3D structures are shaped by mechanical forces, followed by a closer look at patterning processes. We also review the role of forces in homeostatic processes, including cell elimination and wound healing. Finally, we look at the interplay of mechanics and developmental robustness and discuss open questions in the field, as well as novel approaches that will help tackle them in the future.

Anti-Fibrotic Activity of an Antimicrobial Peptide in a Drosophila Model

Fibrotic lesions accompany several pathological conditions, including tumors. We show that expression of a dominant-active form of the Ras oncogene in Drosophila salivary glands (SGs) leads to redistribution of components of the basement membrane (BM) and fibrotic lesions. Similar to several types of mammalian fibrosis, the disturbed BM attracts clot components, including insect transglutaminase and phenoloxidase. SG epithelial cells show reduced apicobasal polarity accompanied by a loss of secretory activity. Both the fibrotic lesions and the reduced cell polarity are alleviated by ectopic expression of the antimicrobial peptide drosomycin (Drs), which also restores the secretory activity of the SGs. In addition to extracellular matrix components, both Drs and F-actin localize to fibrotic lesions.

Fine infrastructure of released and solidified Drosophila larval salivary secretory glue using SEM

The Golgi-derived large secretory granules of Drosophila salivary glands (SGs) constitute the components of the salivary glue secretion (Sgs). The Sgs represents a highly special and unique extracellular composite glue matrix that has not yet been identified outside of Cyclorrhaphous Dipterans. For over half a century, the only major and unambiguously documented function of the larval salivary glands was to produce a large amount of mucinous glue-containing secretory granules that, when released during pupariation, serves to affix the freshly formed puparia to a substrate. Besides initial biochemical characterization of the Sgs proteins and cloning of their corresponding Sgs genes, very little is known about other properties and functions of the Sgs glue. We report here observations on the fine SEM-ultrastructure of the Sgs glue released into to the lumen of SGs, and after it has been expectorated and solidified into the external environment. Surprisingly, in contrast to long held expectations, it appears to be a highly structured bioadhesive mass with an internal spongious to trabecular infrastructure, reflecting the state of its hydratation. We also found that in addition to its cementing properties, it is highly efficient at glueing and trapping microorganisms, and thus may serve a potentially very important immune and defense role. High hydration capacity, the speed by which this glue can dry, uniqueness of its protein composition and spongious infrastructure can provide inspiration for development of potential biomimetics that can attach completely different or incompatible surfaces with high efficiency and strength.

Centrosomal and Non-Centrosomal Microtubule-Organizing Centers (MTOCs) in Drosophila melanogaster

The centrosome is the best-understood microtubule-organizing center (MTOC) and is essential in particular cell types and at specific stages during Drosophila development. The centrosome is not required zygotically for mitosis or to achieve full animal development. Nevertheless, centrosomes are essential maternally during cleavage cycles in the early embryo, for male meiotic divisions, for efficient division of epithelial cells in the imaginal wing disc, and for cilium/flagellum assembly in sensory neurons and spermatozoa. Importantly, asymmetric and polarized division of stem cells is regulated by centrosomes and by the asymmetric regulation of their microtubule (MT) assembly activity. More recently, the components and functions of a variety of non-centrosomal microtubule-organizing centers (ncMTOCs) have begun to be elucidated. Throughout Drosophila development, a wide variety of unique ncMTOCs form in epithelial and non-epithelial cell types at an assortment of subcellular locations. Some of these cell types also utilize the centrosomal MTOC, while others rely exclusively on ncMTOCs. The impressive variety of ncMTOCs being discovered provides novel insight into the diverse functions of MTOCs in cells and tissues. This review highlights our current knowledge of the composition, assembly, and functional roles of centrosomal and non-centrosomal MTOCs in Drosophila.

NM23 proteins: innocent bystanders or local energy boosters for CFTR?

NM23 proteins NDPK-A and -B bind to the cystic fibrosis (CF) protein CFTR in different ways from kinases such as PKA, CK2 and AMPK or linkers to cell calcium such as calmodulin and annexins. NDPK-A (not -B) interacts with CFTR through reciprocal AMPK binding/control, whereas NDPK-B (not -A) binds directly to CFTR. NDPK-B can activate G proteins without ligand-receptor coupling, so perhaps NDPK-B's binding influences energy supply local to a nucleotide-binding site (NBD1) needed for CFTR to function. Curiously, CFTR (ABC-C7) is a member of the ATP-binding cassette (ABC) protein family that does not obey 'clan rules'; CFTR channels anions and is not a pump, regulates disparate processes, is itself regulated by multiple means and is so pleiotropic that it acts as a hub that orchestrates calcium signaling through its consorts such as calmodulin/annexins. Furthermore, its multiple partners make CFTR dance to different tunes in different cellular and subcellular locations as it recycles from the plasma membrane to endosomes. CFTR function in airway apical membranes is inhibited by smoking which has been dubbed 'acquired CF'. CFTR alone among family members possesses a trap for other proteins that it unfurls as a 'fish-net' and which bears consensus phosphorylation sites for many protein kinases, with PKA being the most canonical. Recently, the site of CFTR's commonest mutation has been proposed as a knock-in mutant that alters allosteric control of kinase CK2 by log orders of activity towards calmodulin and other substrates after CFTR fragmentation. This link from CK2 to calmodulin that binds the R region invokes molecular paths that control lumen formation, which is incomplete in the tracheas of some CF-affected babies. Thus, we are poised to understand the many roles of NDPK-A and -B in CFTR function and, especially lumen formation, which is defective in the gut and lungs of many CF babies.

Cellular systems for epithelial invagination

Epithelial invagination is a fundamental module of morphogenesis that iteratively occurs to generate the architecture of many parts of a developing organism. By changing the physical properties such as the shape and/or position of a population of cells, invagination drives processes ranging from reconfiguring the entire body axis during gastrulation, to forming the primordia of the eyes, ears and multiple ducts and glands, during organogenesis. The epithelial bending required for invagination is achieved through a variety of mechanisms involving systems of cells. Here we provide an overview of the different mechanisms, some of which can work in combination, and outline the circumstances in which they apply.

This article is part of the themed issue ‘Systems morphodynamics: understanding the development of tissue hardware’.

Massive excretion of calcium oxalate from late prepupal salivary glands of Drosophila melanogaster demonstrates active nephridial-like anion transport

The Drosophila salivary glands (SGs) were well known for the puffing patterns of their polytene chromosomes and so became a tissue of choice to study sequential gene activation by the steroid hormone ecdysone. One well-documented function of these glands is to produce a secretory glue, which is released during pupariation to fix the freshly formed puparia to the substrate. Over the past two decades SGs have been used to address specific aspects of developmentally-regulated programmed cell death (PCD) as it was thought that they are doomed for histolysis and after pupariation are just awaiting their fate. More recently, however, we have shown that for the first 3-4 h after pupariation SGs undergo tremendous endocytosis and vacuolation followed by vacuole neutralization and membrane consolidation. Furthermore, from 8 to 10 h after puparium formation (APF) SGs display massive apocrine secretion of a diverse set of cellular proteins. Here, we show that during the period from 11 to 12 h APF, the prepupal glands are very active in calcium oxalate (CaOx) extrusion that resembles renal or nephridial excretory activity. We provide genetic evidence that Prestin, a Drosophila homologue of the mammalian electrogenic anion exchange carrier SLC26A5, is responsible for the instantaneous production of CaOx by the late prepupal SGs. Its positive regulation by the protein kinases encoded by fray and wnk lead to increased production of CaOx. The formation of CaOx appears to be dependent on the cooperation between Prestin and the vATPase complex as treatment with bafilomycin A1 or concanamycin A abolishes the production of detectable CaOx. These data demonstrate that prepupal SGs remain fully viable, physiologically active and engaged in various cellular activities at least until early pupal period, that is, until moments prior to the execution of PCD.

Developmental compartments in the larval trachea of Drosophila

The Drosophila tracheal system is a branched tubular network that forms in the embryo by a post-mitotic program of morphogenesis. In third instar larvae (L3), cells constituting the second tracheal metamere (Tr2) reenter the cell cycle. Clonal analysis of L3 Tr2 revealed that dividing cells in the dorsal trunk, dorsal branch and transverse connective branches respect lineage restriction boundaries near branch junctions. These boundaries corresponded to domains of gene expression, for example where cells expressing Spalt, Delta and Serrate in the dorsal trunk meet vein–expressing cells in the dorsal branch or transverse connective. Notch signaling was activated to one side of these borders and was required for the identity, specializations and segregation of border cells. These findings suggest that Tr2 is comprised of developmental compartments and that developmental compartments are an organizational feature relevant to branched tubular networks.

DOI:http://dx.doi.org/10.7554/eLife.08666.001

As a fruit fly develops, its cells may sort themselves into groups according to the type of cell that they will eventually become. Some groups form ‘developmental compartments’ that are separated by boundaries that cells cannot move across. All the descendants of a cell in a compartment will activate the same specific gene (called a ‘selector’ gene) that determines their identity and fate. Similar compartments also form in the developing hindbrains of mammals, but it is not clear how general this mechanism of tissue patterning is.

Fruit fly larvae undergo a physical transformation called metamorphosis to become adult fruit flies. Here, Rao et al. discover that the cells in the developing airways (or trachea) of the larvae at the start of metamorphosis are organised into compartments. At this stage the cells in the trachea start to divide and grow to make the adult tracheal system. The experiments show that these cells do not spread from one main branch of the tracheal system into another. Instead, the cells cluster in locations where the different branches meet on either side of a straight boundary.

The cells on each side of these boundaries activate different genes that regulate their identity and development. For example, cells in one branch of the system switch on a selector gene that makes a protein called Spalt. A pathway known as Notch signaling is activated by cells on the other side of a nearby boundary in a different branch of the tracheal system. This separation of Spalt production and Notch activation establishes a cell communication system that keeps the cells of the different compartments apart.

Rao et al.’s findings reveal a role for the Notch protein in regulating the organization of cells into compartments to form branches in fruit fly airways. A future challenge is to find out if Notch plays a similar role in other branched tissues, such as blood vessels.

DOI:http://dx.doi.org/10.7554/eLife.08666.002

Respiratory metabolism of salivary glands during the late larval and prepupal development of Drosophila melanogaster

During the late larval period, the salivary glands (SG) of Drosophila show a cascade of cytological changes associated with exocytosis and the expectoration of the proteinaceous glue that is used to affix the pupariating larva to a substrate. After puparium formation (APF), SG undergo extensive cytoplasmic vacuolation due to endocytosis, vacuole consolidation and massive apocrine secretion. Here we investigated possible correlations between cytological changes, the puffing pattern in polytene chromosomes and respiratory metabolism of the SG. The carefully staged SG were explanted into small amounts (1 or 2μl) of tissue culture medium. The respiratory metabolism of single or up to 3 pairs of glands was evaluated by recording the rate of O2 consumption using a scanning microrespirographic technique sensitive to subnanoliter volumes of the respiratory O2 or CO2. The recordings were carried out at times between 8h before pupariation (BPF), until 16h APF, at which point the SG completely disintegrate. At the early wandering larval stage (8h BPF), the glands consume 2nl of O2/gland/min (=2500μl O2/g/h). This relatively high metabolic rate decreases down to 1.2-1.3nl of O2 during the endogenous peak in ecdysteroid concentration that culminates around pupariation. The metabolic decline coincides with the exocytosis of the proteinaceous glue. During and shortly after puparium formation, which is accompanied cytologically by intense vacuolation, O2 consumption in the SG temporarily increases to 1.6nl O2/gland/min. After this time, the metabolic rate of the SG decreases downward steadily until 16h APF, when the glands disintegrate and cease to consume oxygen. The SG we analyzed from Drosophila larvae were composed of 134 intrinsic cells, with the average volume of one lobe being 37nl. Therefore, a single SG cell of the wandering larva (with O2 consumption of 2nl/gland/min), consumes each about 16pl of O2/cell/min. A simultaneous analysis of the rate of protein and RNA synthesis in the SG shows a course similar to that found in respiratory metabolism.

JAK/STAT and Hox Dynamic Interactions in an Organogenetic Gene Cascade

Organogenesis is controlled by gene networks activated by upstream selector genes. During development the gene network is activated stepwise, with a sequential deployment of successive transcription factors and signalling molecules that modify the interaction of the elements of the network as the organ forms. Very little is known about the steps leading from the early specification of the cells that form the organ primordium to the moment when a robust gene network is in place. Here we study in detail how a Hox protein induces during early embryogenesis a simple organogenetic cascade that matures into a complex gene network through the activation of feedback and feed forward interaction loops. To address how the network organization changes during development and how the target genes integrate the genetic information it provides, we analyze in Drosophila the induction of posterior spiracle organogenesis by the Hox gene Abdominal-B (Abd-B). Initially, Abd-B activates in the spiracle primordium a cascade of transcription factors and signalling molecules including the JAK/STAT signalling pathway. We find that at later stages STAT activity feeds back directly into Abd-B, initiating the transformation of the Hox cascade into a gene-network. Focusing on crumbs, a spiracle downstream target gene of Abd-B, we analyze how a modular cis regulatory element integrates the dynamic network information set by Abd-B and the JAK/STAT signalling pathway during development. We describe how a Hox induced genetic cascade transforms into a robust gene network during organogenesis due to the repeated interaction of Abd-B and one of its targets, the JAK/STAT signalling cascade. Our results show that in this network STAT functions not just as a direct transcription factor, but also acts as a "counter-repressor", uncovering a novel mode for STAT directed transcriptional regulation.

Organogenesis is controlled by gene networks activated by upstream selector genes. To address how the network organization changes during development and how the target genes integrate the genetic information it provides, we analyze in Drosophila the induction of posterior spiracle organogenesis by the Hox gene Abdominal-B (Abd-B). Initially, Abd-B activates in the spiracle primordium a cascade of transcription factors and signalling molecules including the JAK/STAT pathway. We find that at later stages STAT activity feeds back into Abd-B, initiating the transformation of the Hox cascade into a gene-network. Focusing on a spiracle downstream target gene of Abd-B, we analyze how its cis regulatory elements integrate the dynamic network information set by Abd-B and the JAK/STAT signalling pathway during development. Our results also show that the well known transcription factor STAT can control gene expression as a “counter-repressor”, uncovering an alternative novel mode for STAT directed transcriptional regulation.

The extracellular protease stl functions to inhibit migration of v'ch1 sensory neuron during Drosophila embryogenesis

Proper migration of cells through the dense and complex extracellular matrix (ECM) requires constant restructuring of the ECM to allow cells to move forward in a smooth manner. This restructuring can occur through the action of extracellular enzymes. Among these extracellular enzymes is the ADAMTS (A Disintegrin And Metalloprotease with ThromboSpondin repeats) family of secreted extracellular proteases. Drosophila stl encodes an ADAMTS protease expressed in and around the peripheral nervous system (PNS) during embryogenesis. The absence of stl displayed one specific neuron, the v'ch1 sensory neuron, migrating to its target sooner than in wild type. During normal development, the v'ch1 sensory neuron migrates dorsally at the same time it is extending an axon ventrally toward the CNS. Surprisingly, in the absence of stl, the v'ch1 neuron migrated further dorsally as compared to the wild type at stage 15, but did not migrate past its correct target at stage 16, suggesting a novel role for this extracellular protease in inhibiting migration of this neuron past a certain point.

A genome-wide survey of sexually dimorphic expression of Drosophila miRNAs identifies the steroid hormone-induced miRNA let-7 as a regulator of sexual identity

MiRNAs bear an increasing number of functions throughout development and in the aging adult. Here we address their role in establishing sexually dimorphic traits and sexual identity in male and female Drosophila. Our survey of miRNA populations in each sex identifies sets of miRNAs differentially expressed in male and female tissues across various stages of development. The pervasive sex-biased expression of miRNAs generally increases with the complexity and sexual dimorphism of tissues, gonads revealing the most striking biases. We find that the male-specific regulation of the X chromosome is relevant to miRNA expression on two levels. First, in the male gonad, testis-biased miRNAs tend to reside on the X chromosome. Second, in the soma, X-linked miRNAs do not systematically rely on dosage compensation. We set out to address the importance of a sex-biased expression of miRNAs in establishing sexually dimorphic traits. Our study of the conserved let-7-C miRNA cluster controlled by the sex-biased hormone ecdysone places let-7 as a primary modulator of the sex-determination hierarchy. Flies with modified let-7 levels present doublesex-related phenotypes and express sex-determination genes normally restricted to the opposite sex. In testes and ovaries, alterations of the ecdysone-induced let-7 result in aberrant gonadal somatic cell behavior and non-cell-autonomous defects in early germline differentiation. Gonadal defects as well as aberrant expression of sex-determination genes persist in aging adults under hormonal control. Together, our findings place ecdysone and let-7 as modulators of a somatic systemic signal that helps establish and sustain sexual identity in males and females and differentiation in gonads. This work establishes the foundation for a role of miRNAs in sexual dimorphism and demonstrates that similar to vertebrate hormonal control of cellular sexual identity exists in Drosophila.

Anatomy, biogenesis and regeneration of salivary glands

An overview of the anatomy and biogenesis of salivary glands is important in order to understand the physiology, functions and disorders associated with saliva. A major disorder of salivary glands is salivary hypofunction and resulting xerostomia, or dry mouth, which affects hundreds of thousands of patients each year who suffer from salivary gland diseases or undergo head and neck cancer treatment. There is currently no curative therapy for these patients. To improve these patients' quality of life, new therapies are being developed based on findings in salivary gland cell and developmental biology. Here we discuss the anatomy and biogenesis of the major human salivary glands and the rodent submandibular gland, which has been used extensively as a research model. We also include a review of recent research on the identification and function of stem cells in salivary glands, and the emerging field of research suggesting that nerves play an instructive role during development and may be essential for adult gland repair and regeneration. Understanding the molecular mechanisms involved in gland biogenesis provides a template for regenerating, repairing or reengineering diseased or damaged adult human salivary glands. We provide an overview of 3 general approaches currently being developed to regenerate damaged salivary tissue, including gene therapy, stem cell-based therapy and tissue engineering. In the future, it may be that a combination of all three will be used to repair, regenerate and reengineer functional salivary glands in patients to increase the secretion of their saliva, the focus of this monograph.

Tip cells: master regulators of tubulogenesis?

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Single tip cells or groups of leading cells develop at the forefront of growing tissues.

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Tip cells regulate tubule growth and morphogenesis.

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Tip cells develop distinctive patterns of gene expression and specialised characteristics.

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Tip cells are required for health and may be involved in the progression of cancer.

The normal development of an organ depends on the coordinated regulation of multiple cell activities. Focusing on tubulogenesis, we review the role of specialised cells or groups of cells that are selected from within tissue primordia and differentiate at the outgrowing tips or leading edge of developing tubules. Tip or leading cells develop distinctive patterns of gene expression that enable them to act both as sensors and transmitters of intercellular signalling. This enables them to explore the environment, respond to both tissue intrinsic signals and extrinsic cues from surrounding tissues and to regulate the behaviour of their neighbours, including the setting of cell fate, patterning cell division, inducing polarity and promoting cell movement and cell rearrangements by neighbour exchange. Tip cells are also able to transmit mechanical tension to promote tissue remodelling and, by interacting with the extracellular matrix, they can dictate migratory pathways and organ shape. Where separate tubular structures fuse to form networks, as in the airways of insects or the vascular system of vertebrates, specialised fusion tip cells act to interconnect disparate elements of the developing network. Finally, we consider their importance in the maturation of mature physiological function and in the development of disease.

Hox targets and cellular functions

Hox genes are a group of genes that specify structures along the anteroposterior axis in bilaterians. Although in many cases they do so by modifying a homologous structure with a different (or no) Hox input, there are also examples of Hox genes constructing new organs with no homology in other regions of the body. Hox genes determine structures though the regulation of targets implementing cellular functions and by coordinating cell behavior. The genetic organization to construct or modify a certain organ involves both a genetic cascade through intermediate transcription factors and a direct regulation of targets carrying out cellular functions. In this review I discuss new data from genome-wide techniques, as well as previous genetic and developmental information, to describe some examples of Hox regulation of different cell functions. I also discuss the organization of genetic cascades leading to the development of new organs, mainly using Drosophila melanogaster as the model to analyze Hox function.

Common origin of insect trachea and endocrine organs from a segmentally repeated precursor

Segmented organisms have serially repeated structures [1] that become specialized in some segments [2]. We show here that the Drosophila corpora allata, prothoracic glands, and trachea have a homologous origin and can convert into each other. The tracheal epithelial tubes develop from ten trunk placodes [3, 4], and homologous ectodermal cells in the maxilla and labium form the corpora allata and the prothoracic glands. The early endocrine and trachea gene networks are similar, with STAT and Hox genes inducing their activation. The initial invagination of the trachea and the endocrine primordia is identical, but activation of Snail in the glands induces an epithelial-mesenchymal transition (EMT), after which the corpora allata and prothoracic gland primordia coalesce and migrate dorsally, joining the corpora cardiaca to form the ring gland. We propose that the arthropod ectodermal endocrine glands and respiratory organs arose through an extreme process of divergent evolution from a metameric repeated structure.

Regulatory interactions between androgens, Hoxb5, and TGF β signaling in murine lung development

Androgens enhance airway branching but delay alveolar maturation contributing to increased respiratory morbidity in prematurely born male infants. Hoxb5 protein positively regulates airway branching in developing lung. In other organs, androgen regulation intersects with Hox proteins and TGFβ-SMAD signaling, but these interactions have not been studied in the lung. We hypothesized that androgen alteration of airway branching early in lung development requires Hoxb5 expression and that these androgen-Hoxb5 interactions occur partially through regional changes in TGFβ signaling. To evaluate acute effects of androgen and TGFβ on Hoxb5, E11 whole fetal mouse lungs were cultured with dihydrotestosterone (DHT) with/without Hoxb5 siRNA or TGFβ inhibitory antibody. Chronic in utero DHT exposure was accomplished by exposing pregnant mice to DHT (subcutaneous pellet) from E11 to E18. DHT's ability to enhance airway branching and alter phosphorylated SMAD2 cellular localization was partially dependent on Hoxb5. Hoxb5 inhibition also changed the cellular distribution of SMAD7 protein. Chronic in utero DHT increased Hoxb5 and altered SMAD7 mesenchymal localization. TGFβ inhibition enhanced airway branching, and Hoxb5 protein cellular localization was more diffuse. We conclude that DHT controls lung airway development partially through modulation of Hoxb5 protein expression and that this level of regulation involves interactions with TGFβ signaling.

The secreted AdamTS-A metalloprotease is required for collective cell migration

Members of the ADAMTS family of secreted metalloproteases play crucial roles in modulating the extracellular matrix (ECM) in development and disease. Here, we show that ADAMTS-A, the Drosophila ortholog of human ADAMTS 9 and ADAMTS 20, and of C. elegans GON-1, is required for cell migration during embryogenesis. AdamTS-A is expressed in multiple migratory cell types, including hemocytes, caudal visceral mesoderm (CVM), the visceral branch of the trachea (VBs) and the secretory portion of the salivary gland (SG). Loss of AdamTS-A causes defects in germ cell, CVM and VB migration and, depending on the tissue, AdamTS-A functions both autonomously and non-autonomously. In the highly polarized collective of the SG epithelium, loss of AdamTS-A causes apical surface irregularities and cell elongation defects. We provide evidence that ADAMTS-A is secreted into the SG lumen where it functions to release cells from the apical ECM, consistent with the defects observed in AdamTS-A mutant SGs. We show that loss of the apically localized protocadherin Cad99C rescues the SG defects, suggesting that Cad99C serves as a link between the SG apical membrane and the secreted apical ECM component(s) cleaved by ADAMTS-A. Our analysis of AdamTS-A function in the SG suggests a novel role for ADAMTS proteins in detaching cells from the apical ECM, facilitating tube elongation during collective cell migration.

Fosmid-based structure-function analysis reveals functionally distinct domains in the cytoplasmic domain of Drosophila crumbs

The evolutionarily conserved transmembrane protein Crumbs is required for epithelial polarity and morphogenesis in the embryo, control of tissue size in imaginal discs and morphogenesis of photoreceptor cells, and prevents light-dependent retinal degeneration. The small cytoplasmic domain contains two highly conserved regions, a FERM (i.e., protein 4.1/ezrin/radixin/moesin)-binding and a PDZ (i.e., postsynaptic density/discs large/ZO-1)-binding domain. Using a fosmid-based transgenomic approach, we analyzed the role of the two domains during invagination of the tracheae and the salivary glands in the Drosophila embryo. We provide data to show that the PDZ-binding domain is essential for the maintenance of cell polarity in both tissues. In contrast, in embryos expressing a Crumbs protein with an exchange of a conserved Tyrosine residue in the FERM-binding domain to an Alanine, both tissues are internalized, despite some initial defects in apical constriction, phospho-Moesin recruitment, and coordinated invagination movements. However, at later stages these embryos fail to undergo dorsal closure, germ band retraction, and head involution. In addition, frequent defects in tracheal fusion were observed. These results suggest stage and/or tissue specific binding partners. We discuss the power of this fosmid-based system for detailed structure-function analyses in comparison to the UAS/Gal4 system.

Dynamics of branched tissue assembly

The assembly of cells into tissues is a complex process controlled by numerous signaling pathways to ensure the fidelity of the final structure. Tissue assembly is also very dynamic, as exemplified by the formation of branched organs. Here we present two examples of tissue assembly in branched systems that highlight this dynamic nature: formation of the tracheal network in Drosophila melanogaster and the ducts of the mammary gland in mice. Extension of the branches during tracheal development is a stereotyped process that produces identical organ geometries across individuals, whereas elongation of the ducts of the pubertal mammary gland is a non-stereotyped process that produces unique patterns. By studying these two organs, we can begin to understand the dynamic nature of development of other stereotyped and non-stereotyped branching systems, including the lung, kidney, and salivary gland.

Drosophila as a model for epithelial tube formation

Epithelial tubular organs are essential for life in higher organisms and include the pancreas and other secretory organs that function as biological factories for the synthesis and delivery of secreted enzymes, hormones, and nutrients essential for tissue homeostasis and viability. The lungs, which are necessary for gas exchange, vocalization, and maintaining blood pH, are organized as highly branched tubular epithelia. Tubular organs include arteries, veins, and lymphatics, high-speed passageways for delivery and uptake of nutrients, liquids, gases, and immune cells. The kidneys and components of the reproductive system are also epithelial tubes. Both the heart and central nervous system of many vertebrates begin as epithelial tubes. Thus, it is not surprising that defects in tube formation and maintenance underlie many human diseases. Accordingly, a thorough understanding how tubes form and are maintained is essential to developing better diagnostics and therapeutics. Among the best-characterized tubular organs are the Drosophila salivary gland and trachea, organs whose relative simplicity have allowed for in depth analysis of gene function, yielding key mechanistic insight into tube initiation, remodeling and maintenance. Here, we review our current understanding of salivary gland and trachea formation - highlighting recent discoveries into how these organs attain their final form and function.

Trachealess (Trh) regulates all tracheal genes during Drosophila embryogenesis

The Drosophila trachea is a branched tubular epithelia that transports oxygen and other gases. trachealess (trh), which encodes a bHLH-PAS transcription factor, is among the first genes to be expressed in the cells that will form the trachea. In the absence of trh, tracheal cells fail to invaginate to form tubes and remain on the embryo surface. Expression of many tracheal-specific genes depends on trh, but all of the known targets have relatively minor phenotypes compared to loss of trh, suggesting that there are additional targets. To identify uncharacterized transcriptional targets of Trh and to further understand the role of Trh in embryonic tracheal formation, we performed an in situ hybridization screen using a library of ~100 tracheal-expressed genes identified by the Berkeley Drosophila Genome Project (BDGP). Surprisingly, expression of every tracheal gene we tested was dependent on Trh, suggesting a major role for Trh in activation and maintenance of tracheal gene expression. A re-examination of the interdependence of the known early-expressed transcription factors, including trh, ventral veinless (vvl) and knirps/knirps-related (kni/knrl), suggests a new model for how gene expression is controlled in the trachea, with trh regulating expression of vvl and kni, but not vice versa. A pilot screen for the targets of Vvl and Kni/Knrl revealed that Vvl and Kni have only minor roles compared to Trh. Finally, genome-wide microarray experiments identified additional Trh targets and revealed that a variety of biological processes are affected by the loss of trh.

Metastasis Update: Human Prostate Carcinoma Invasion via Tubulogenesis

This paper proposes that human prostate carcinoma primarily invades as a cohesive cell collective through a mechanism similar to embryonic tubulogenesis, instead of the popular epithelial-mesenchymal transformation (EMT) model. Evidence supporting a tubulogenesis model is presented, along with suggestions for additional research. Additionally, observations documenting cell adhesion molecule changes in tissue and stromal components are reviewed, allowing for comparisons between the current branching morphogenesis models and the tubulogenesis model. Finally, the implications of this model on prevailing views of therapeutic and diagnostic strategies for aggressive prostatic disease are considered.

Branch formation during organ development

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

Regulated Crb accumulation controls apical constriction and invagination in Drosophila tracheal cells

Many epithelial tissues undergo extensive remodelling during morphogenesis. How their epithelial features, such as apicobasal polarity or adhesion, are maintained and remodelled and how adhesion and polarity proteins contribute to morphogenesis are two important questions in development. Here, we approach these issues by investigating the role of the apical determinant protein Crumbs (Crb) during the morphogenesis of the embryonic Drosophila tracheal system. Crb accumulates differentially throughout tracheal development and is required for different tracheal events. The earliest requirement for Crb is for tracheal invagination, which is preceded by an enhanced accumulation of Crb in the invagination domain. There, Crb, acting in parallel with the epidermal growth factor receptor (Egfr) pathway, is required for tracheal cell apical constriction and for organising an actomyosin complex, which we propose is mediated by Crb recruitment of moesin (Moe). The ability of a Crb isoform unable to rescue polarity in crb mutants to otherwise rescue their invagination phenotype, and the converse inability of a FERM-binding domain mutant Crb to rescue faulty invagination, support our hypothesis that it is the absence of Crb-dependent Moe enrichment, and not the polarity defect, that mainly underlies the crb invagination phenotype. This hypothesis is supported by the phenotype of lethal giant larvae (lgl); crb double mutants. These results unveil a link between Crb and the organisation of the actin cytoskeleton during morphogenesis.

The CrebA/Creb3-like transcription factors are major and direct regulators of secretory capacity

CrebA up-regulates expression of both the general protein machinery required in all cells for secretion and genes encoding cell type–specific secreted components.

Secretion occurs in all cells, with relatively low levels in most cells and extremely high levels in specialized secretory cells, such as those of the pancreas, salivary, and mammary glands. How secretory capacity is selectively up-regulated in specialized secretory cells is unknown. Here, we find that the CrebA/Creb3-like family of bZip transcription factors functions to up-regulate expression of both the general protein machinery required in all cells for secretion and of cell type–specific secreted proteins. Drosophila CrebA directly binds the enhancers of secretory pathway genes and is both necessary and sufficient to activate expression of every secretory pathway component gene examined thus far. Microarray profiling reveals that CrebA also up-regulates expression of genes encoding cell type–specific secreted components. Finally, we found that the human CrebA orthologues, Creb3L1 and Creb3L2, have the ability to up-regulate the secretory pathway in nonsecretory cell types.

Staying alive: dalmation mediated blocking of apoptosis is essential for tissue maintenance

In an EMS screen for mutations disrupting tracheal development, we identified new alleles of the dalmation (dmt) gene, which had previously been shown to affect peripheral nervous system (PNS) development. Here, we demonstrate that dmt loss results in programmed cell death, disrupting PNS patterning and leading to large gaps in the salivary duct and trachea. Dmt loss results in increased expression of the proapoptotic regulator genes head involution defective (hid) and reaper (rpr), and deletion of these genes or tissue-specific expression of the baculoviral apoptotic inhibitor P35 rescues the dmt defects. dmt is also required to protect cells from irradiation induced expression of hid and rpr during the irradiation resistant stage, which begins as cells become irreversibly committed to their final fates. Thus, we propose that Dmt keeps cells alive by blocking activation of hid and rpr as cells become irreversibly committed.

The cytoskeletal regulator zyxin is required for viability in Drosophila melanogaster

The zyxin family of proteins function as cytoskeletal regulators in adhesion, actin assembly, and cell motility. Though fibroblasts derived from zyxin-null mice show striking defects in motility and response to mechanical stimuli, the mice are viable and fertile. In Drosophila melanogaster, the family is represented by a single homologue, Zyx102. To study the role of zyxin during development, we generated a zyx102 RNA-interference transgenic line that allows for the conditional knockdown of Zyx102. When UAST-zyx102-dsRNAi expression is driven broadly by Actin5C-GAL4, loss of Zyx102 results in lethality during the pharate adult stage, a narrow developmental window during which the fly must molt, resorb molting fluid, fill adult trachea with air, and execute a behavioral program to eclose. Zyx102 knockdown animals attempt to emerge, but their adult trachea do not fill with air. If dissected from the pupal case, knockdown individuals appear morphologically normal, but remain inviable.

Function and specificity of synthetic Hox transcription factors in vivo

Homeotic (Hox) genes encode transcription factors that confer segmental identity along the anteroposterior axis of the embryo. However the molecular mechanisms underlying Hox-mediated transcription and the differential requirements for specificity in the regulation of the vast number of Hox-target genes remain ill-defined. Here we show that synthetic Sex combs reduced (Scr) genes that encode the Scr C terminus containing the homedomain (HD) and YPWM motif (Scr-HD) are functional in vivo. Synthetic Scr-HD peptides can induce ectopic salivary glands in the embryo and homeotic transformations in the adult fly, act as transcriptional activators and repressors during development, and participate in protein-protein interactions. Their transformation capacity was found to be enhanced over their full-length counterpart and mutations known to transform the full-length protein into constitutively active or inactive variants behaved accordingly in the synthetic peptides. Our results show that synthetic Scr-HD genes are sufficient for homeotic function in Drosophila and suggest that the N terminus of Scr has a role in transcriptional potency, rather than specificity. We also demonstrate that synthetic peptides behave largely in a predictable way, by exhibiting Scr-specific phenotypes throughout development, which makes them an important tool for synthetic biology.

Formins in development: orchestrating body plan origami

Formins, proteins defined by the presence of an FH2 domain and their ability to nucleate linear F-actin de novo, play a key role in the regulation of the cytoskeleton. Initially thought to primarily regulate actin, recent studies have highlighted a role for formins in the regulation of microtubule dynamics, and most recently have uncovered the ability of some formins to coordinate the organization of both the microtubule and actin cytoskeletons. While biochemical analyses of this family of proteins have yielded many insights into how formins regulate diverse cytoskeletal reorganizations, we are only beginning to appreciate how and when these functional properties are relevant to biological processes in a developmental or organismal context. Developmental genetic studies in fungi, Dictyostelium, vertebrates, plants and other model organisms have revealed conserved roles for formins in cell polarity, actin cable assembly and cytokinesis. However, roles have also been discovered for formins that are specific to particular organisms. Thus, formins perform both global and specific functions, with some of these roles concurring with previous biochemical data and others exposing new properties of formins. While not all family members have been examined across all organisms, the analyses to date highlight the significance of the flexibility within the formin family to regulate a broad spectrum of diverse cytoskeletal processes during development.

Serrano (sano) functions with the planar cell polarity genes to control tracheal tube length

Epithelial tubes are the functional units of many organs, and proper tube geometry is crucial for organ function. Here, we characterize serrano (sano), a novel cytoplasmic protein that is apically enriched in several tube-forming epithelia in Drosophila, including the tracheal system. Loss of sano results in elongated tracheae, whereas Sano overexpression causes shortened tracheae with reduced apical boundaries. Sano overexpression during larval and pupal stages causes planar cell polarity (PCP) defects in several adult tissues. In Sano-overexpressing pupal wing cells, core PCP proteins are mislocalized and prehairs are misoriented; sano loss or overexpression in the eye disrupts ommatidial polarity and rotation. Importantly, Sano binds the PCP regulator Dishevelled (Dsh), and loss or ectopic expression of many known PCP proteins in the trachea gives rise to similar defects observed with loss or gain of sano, revealing a previously unrecognized role for PCP pathway components in tube size control.

Tubular organ formation is a ubiquitous process required to sustain life in multicellular organisms. In this study, we focused on the tracheal system of the fruit fly, Drosophila melanogaster, and identified Serrano (Sano) as a novel protein expressed in several embryonic tubular organs, including trachea. sano loss results in over-elongated trachea, whereas Sano overexpression causes shortened trachea, suggesting that sano is required for proper tracheal tube length. Interestingly, Sano overexpression results in typical planar cell polarity (PCP) defects in many adult tissues and pupal wing cells. The PCP pathway is highly conserved from flies to mammals and it has been known to control cell polarity within the plane of epithelial tissues. Importantly, we found that Sano binds Dishevelled (Dsh), a key PCP regulator, and loss or ectopic expression of many known PCP proteins in the trachea give rise to similar defects observed with loss or gain of sano, suggesting a new role for the PCP genes in tube length control. Interestingly, the changes in tube length and PCP defects in the wing were linked to changes in apical domain size, suggesting that Sano and the PCP components affect either membrane recycling and/or the linkage of the membrane to the cytoskeleton.

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

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

Apical secretion in epithelial tubes of the Drosophila embryo is directed by the Formin-family protein Diaphanous

Apical localization of filamentous actin (F-actin) is a common feature of epithelial tubes in multicellular organisms. However, its origins and function are not known. We demonstrate that the Diaphanous (Dia)/Formin actin-nucleating factor is required for generation of apical F-actin in diverse types of epithelial tubes in the Drosophila embryo. Dia itself is apically localized both at the RNA and protein levels, and apical localization of its activators, including Rho1 and two guanine exchange factor proteins (Rho-GEFs), contributes to its activity. In the absence of apical actin polymerization, apical-basal polarity and microtubule organization of tubular epithelial cells remain intact; however, secretion through the apical surface to the lumen of tubular organs is blocked. Apical secretion also requires the Myosin V (MyoV) motor, implying that secretory vesicles are targeted to the apical membrane by MyoV-based transport, along polarized actin filaments nucleated by Dia. This mechanism allows efficient utilization of the entire apical membrane for secretion.

Drosophila convoluted/dALS is an essential gene required for tracheal tube morphogenesis and apical matrix organization

Insulin-like growth factors (IGFs) control cell and organism growth through evolutionarily conserved signaling pathways. The mammalian acid-labile subunit (ALS) is a secreted protein that complexes with IGFs to modulate their activity. Recent work has shown that a Drosophila homolog of ALS, dALS, can also complex with and modulate the activity of a Drosophila IGF. Here we report the first mutations in the gene encoding dALS. Unexpectedly, we find that these mutations are allelic to a previously described mutation in convoluted (conv), a gene required for epithelial morphogenesis. In conv mutants, the tubes of the Drosophila tracheal system become abnormally elongated without altering tracheal cell number. conv null mutations cause larval lethality, but do not disrupt several processes required for tracheal tube size control, including septate junction formation, deposition of a lumenal/apical extracellular matrix, and lumenal secretion of Vermiform and Serpentine, two putative matrix-modifying proteins. Clearance of lumenal matrix and subcellular localization of clathrin also appear normal in conv mutants. However, we show that Conv/dALS is required for the dynamic organization of the transient lumenal matrix and normal structure of the cuticle that lines the tracheal lumen. These and other data suggest that the Conv/dALS-dependent tube size control mechanism is distinct from other known processes involved in tracheal tube size regulation. Moreover, we present evidence indicating that Conv/dALS has a novel, IGF-signaling independent function in tracheal morphogenesis.

Diverse roles of E-cadherin in the morphogenesis of the submandibular gland: insights into the formation of acinar and ductal structures

The formation of acinar and ductal structures during epithelial tissue branching morphogenesis is not well understood. We report that in the mouse submandibular gland (SMG), acinar and ductal cell fates are determined early in embryonic morphogenesis with E-cadherin playing pivotal roles in development. We identified two morphologically distinct cell populations at the single bud stage, destined for different functions. The outer layer of columnar cells with organized E-cadherin junctions expressed the neonatal acinar marker B1 by E13.5, demonstrating their acinar fate. The interior cells initially lacked distinct E-cadherin junctions, but with morphogenesis formed cytokeratin 7 (K7) -positive ductal structures with organized E-cadherin junctions and F-actin filaments. Inhibition of E-cadherin function with either siRNA or function blocking antibody caused extensive apoptosis of ductal cells and aberrantly dilated lumens, providing the first evidence that E-cadherin regulates ductal lumen formation during branching morphogenesis of the salivary gland.

Cellular and molecular mechanisms underlying the formation of biological tubes

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

Tissue remodeling: a mating-induced differentiation program for the Drosophila oviduct

BACKGROUND

In both vertebrates and invertebrates, the oviduct is an epithelial tube surrounded by visceral muscles that serves as a conduit for gamete transport between the ovary and uterus. While Drosophila is a model system for tubular organ development, few studies have addressed the development of the fly's oviduct. Recent studies in Drosophila have identified mating-responsive genes and proteins whose levels in the oviduct are altered by mating. Since many of these molecules (e.g. Muscle LIM protein 84B, Coracle, Neuroglian) have known roles in the differentiation of muscle and epithelia of other organs, mating may trigger similar differentiation events in the oviduct. This led us to hypothesize that mating mediates the last stages of oviduct differentiation in which organ-specific specializations arise.

RESULTS

Using electron- and confocal-microscopy we identified tissue-wide post-mating changes in the oviduct including differentiation of cellular junctions, remodeling of extracellular matrix, increased myofibril formation, and increased innervation. Analysis of once- and twice-mated females reveals that some mating-responsive proteins respond only to the first mating, while others respond to both matings.

CONCLUSION

We uncovered ultrastructural changes in the mated oviduct that are consistent with the roles that mating-responsive proteins play in muscle and epithelial differentiation elsewhere. This suggests that mating triggers the late differentiation of the oviduct. Furthermore, we suggest that mating-responsive proteins that respond only to the first mating are involved in the final maturation of the oviduct while proteins that remain responsive to later matings are also involved in maintenance and ongoing function of the oviduct. Taken together, our results establish the oviduct as an attractive system to address mechanisms that regulate the late stages of differentiation and maintenance of a tubular organ.