Dual origin of tissue-specific progenitor cells in Drosophila tracheal remodeling

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.

Polyploidy-associated autophagy promotes larval tracheal histolysis at Drosophila metamorphosis

Polyploidy is an extended phenomenon in biology. However, its physiological significance and whether it defines specific cell behaviors is not well understood. Here we study its connection to macroautophagy/autophagy, using the larval respiratory system of Drosophila as a model. This system comprises cells with the same function yet with notably different ploidy status, namely diploid progenitors and their polyploid larval counterparts, the latter destined to die during metamorphosis. We identified an association between polyploidy and autophagy and found that higher endoreplication status correlates with elevated autophagy. Finally, we report that tissue histolysis in the trachea during Drosophila metamorphosis is mediated by autophagy, which triggers the apoptosis of polyploid cells.Abbreviations: APF: after pupa formation; Atg: autophagy related; btl: breathless; CycE: Cyclin E; DT: dorsal trunk; fzr: fizzy-related; L3: larval stage 3; PBS: phosphate-buffered saline; RI: RNAi; Tr: tracheal metamere; yki: yorkie.

The tracheal immune system of insects - A blueprint for understanding epithelial immunity

The unique design of respiratory organs in multicellular organisms makes them prone to infection by pathogens. To cope with this vulnerability, highly effective local immune systems evolved that are also operative in the tracheal system of insects. Many pathogens and parasites (including viruses, bacteria, fungi, and metazoan parasites) colonize the trachea or invade the host via this route. Currently, only two modules of the tracheal immune system have been characterized in depth: 1) Immune deficiency pathway-mediated activation of antimicrobial peptide gene expression and 2) local melanization processes that protect the structure from wounding. There is an urgent need to increase our understanding of the architecture of tracheal immune systems, especially regarding those mechanisms that enable the maintenance of immune homeostasis. This need for new studies is particularly exigent for species other than Drosophila.

POUM2 homeostasis regulates intimal remodeling and cells fate in the anterior silk gland of the silkworm

Tissue/organ remodeling and cells fate determination play key roles in the life cycle of animals. However, they are still poorly understood in insects, especially in the silkworm. The anterior silk gland (ASG) of the silkworm is essential for the formation and performance of silk fibers, but the regulatory mechanism of ASG remodeling and cells fate determination is less known. Here we found that silencing of POUM2 caused shorter ASG length, intimal structural defects, silkworm spinning failure, and the resultant naked pupae death, but cells number was not affected. Cells staining showed that DNA endoreduplication was not affected in the ASG. Transmission electron microscopy and chitin staining showed cuticle proteins and chitin were greatly reduced in the ASG during the molting period. Transcriptional analysis showed the expression profiles of cuticle proteins and chitin synthase were similar to that of POUM2 during the molting period, and POUM2 down-regulation reduced the expression of cuticle proteins, chitin synthase, autophagy and apoptosis-related genes. While the phenotype resulting from POUM2 over-expression was similar to that of POUM2 down-regulation. Cells staining revealed marked cells apoptosis with cells number reduction and inhibition of DNA endoreduplication in the ASG. Transcriptional analysis showed the expression of autophagy and apoptosis-related genes, and some cuticle proteins and chitin synthase were significantly up-regulated. The results suggest that POUM2 homeostasis regulates ASG intimal remodeling and cells fate, thus affecting ASG development, silkworm spinning and metamorphosis. Our studies not only offer potential molecular targets for genetic improvement of silk performance and molecular breeding of the silkworm, but also provide new insights into POU factor-mediated tissue remodeling and cells fate determination in insects.

Metabolic control of progenitor cell propagation during Drosophila tracheal remodeling

Adult progenitor cells in the trachea of Drosophila larvae are activated and migrate out of niches when metamorphosis induces tracheal remodeling. Here we show that in response to metabolic deficiency in decaying tracheal branches, signaling by the insulin pathway controls the progenitor cells by regulating Yorkie (Yki)-dependent proliferation and migration. Yki, a transcription coactivator that is regulated by Hippo signaling, promotes transcriptional activation of cell cycle regulators and components of the extracellular matrix in tracheal progenitor cells. These findings reveal that regulation of Yki signaling by the insulin pathway governs proliferation and migration of tracheal progenitor cells, thereby identifying the regulatory mechanism by which metabolic depression drives progenitor cell activation and cell division that underlies tracheal remodeling.

Tracheal remodeling is a key step during Drosophila metamorphosis. Here they report that tracheal progenitor cells are activated through Yorkie-dependent proliferation and migration, which is induced by metabolic deficit and insulin signaling.

Drosophila larval epidermal cells only exhibit epidermal aging when they persist to the adult stage

Holometabolous insects undergo a complete transformation of the body plan from the larval to the adult stage. In Drosophila, this transformation includes replacement of larval epidermal cells (LECs) by adult epidermal cells (AECs). AECs in Drosophila undergo a rapid and stereotyped aging program where they lose both cell membranes and nuclei. Whether LECs are capable of undergoing aging in a manner similar to AECs remains unknown. Here, we address this question in two ways. First, we looked for hallmarks of epidermal aging in larvae that have a greatly extended third instar and/or carry mutations that would cause premature epidermal aging at the adult stage. Such larvae, irrespective of genotype, did not show any of the signs of epidermal aging observed in the adult. Second, we developed a procedure to effect a heterochronic persistence of LECs into the adult epidermal sheet. Lineage tracing verified that presumptive LECs in the adult epidermis are not derived from imaginal epidermal histoblasts. LECs embedded within the adult epidermal sheet undergo clear signs of epidermal aging; they form multinucleate cells with each other and with the surrounding AECs. The incidence of adult cells with mixed AEC nuclei (small) and persistent LEC nuclei (large) increased with age. Our data reveals that epidermal aging in holometabolous Drosophila is a stage-specific phenomenon and that the capacity of LECs to respond to aging signals does exist.

Systemic and local effect of the Drosophila headcase gene and its role in stress protection of Adult Progenitor Cells

During the development of a holometabolous insect such as Drosophila, specific group of cells in the larva survive during metamorphosis, unlike the other larval cells, and finally give rise to the differentiated adult structures. These cells, also known as Adult Progenitor Cells (APCs), maintain their multipotent capacity, differentially respond to hormonal and nutritional signals, survive the intrinsic and environmental stress and respond to the final differentiation cues. However, not much is known about the specific molecular mechanisms that account for their unique characteristics. Here we show that a specific Drosophila APC gene, headcase (hdc), has a dual role in the normal development of these cells. It acts at a systemic level by controlling the hormone ecdysone in the prothoracic gland and at the same time it acts locally as a tissue growth suppressor in the APC clusters, where it modulates the activity of the TOR pathway and promotes their survival by contributing in the regulation of the Unfolded Protein Response. We also show that hdc provides protection against stress in the APCs and that its ectopic expression in cells that do not usually express hdc can confer these cells with an additional stress protection. Hdc is the founding member of a group of homolog proteins identified from C. elegans to humans, where has been found associated with cancer progression. The finding that the Drosophila hdc is specifically expressed in progenitor cells and that it provides protection against stress opens up a new hypothesis to be explored regarding the role of the human Heca and its contribution to carcinogenesis.

Rare Pulmonary Neuroendocrine Cells Are Stem Cells Regulated by Rb, p53, and Notch

Pulmonary neuroendocrine (NE) cells are neurosensory cells sparsely distributed throughout the bronchial epithelium, many in innervated clusters of 20-30 cells. Following lung injury, NE cells proliferate and generate other cell types to promote epithelial repair. Here, we show that only rare NE cells, typically 2-4 per cluster, function as stem cells. These fully differentiated cells display features of classical stem cells. Most proliferate (self-renew) following injury, and some migrate into the injured area. A week later, individual cells, often just one per cluster, lose NE identity (deprogram), transit amplify, and reprogram to other fates, creating large clonal repair patches. Small cell lung cancer (SCLC) tumor suppressors regulate the stem cells: Rb and p53 suppress self-renewal, whereas Notch marks the stem cells and initiates deprogramming and transit amplification. We propose that NE stem cells give rise to SCLC, and transformation results from constitutive activation of stem cell renewal and inhibition of deprogramming.

A hemipteran insect reveals new genetic mechanisms and evolutionary insights into tracheal system development

The diversity in the organization of the tracheal system is one of the drivers of insect evolutionary success; however, the genetic mechanisms responsible are yet to be elucidated. Here, we highlight the advantages of utilizing hemimetabolous insects, such as the milkweed bug Oncopeltus fasciatus, in which the final adult tracheal patterning can be directly inferred by examining its blueprint in embryos. By reporting the expression patterns, functions, and Hox gene regulation of trachealess (trh), ventral veinless (vvl), and cut (ct), key genes involved in tracheal development, this study provides important insights. First, Hox genes function as activators, modifiers, and suppressors of trh expression, which in turn results in a difference between the thoracic and abdominal tracheal organization. Second, spiracle morphogenesis requires the input of both trh and ct, where ct is positively regulated by trh As Hox genes regulate trh, we can now mechanistically explain the previous observations of their effects on spiracle formation. Third, the default state of vvl expression in the thorax, in the absence of Hox gene expression, features three lateral cell clusters connected to ducts. Fourth, the exocrine scent glands express vvl and are regulated by Hox genes. These results extend previous findings [Sánchez-Higueras et al., 2014], suggesting that the exocrine glands, similar to the endocrine, develop from the same primordia that give rise to the trachea. The presence of such versatile primordia in the miracrustacean ancestor could account for the similar gene networks found in the glandular and respiratory organs of both insects and crustaceans.

Dual role of FGF in proliferation and endoreplication of Drosophila tracheal adult progenitor cells

Adult progenitor cells activation is a key event in the formation of adult organs. In Drosophila, formation of abdominal adult trachea depends on the specific activation of tracheal adult progenitors (tracheoblasts) at the Tr4 and Tr5 spiracular branches. Proliferation of these tracheoblasts generates a pool of tracheal cells that migrate toward the posterior part of the trachea by the activation of the branchless/fibroblast growth factor (Bnl/FGF) signaling to form the abdominal adult trachea. Here, we show that, in addition to migration, Bnl/FGF signaling, mediated by the transcription factor Pointed, is also required for tracheoblast proliferation. This tracheoblast activation relies on the expression of the FGF ligand bnl in their nearby branches. Finally, we show that, in the absence of the transcription factor Cut (Ct), Bnl/FGF signaling induces endoreplication of tracheoblasts partially by promoting fizzy-related expression. Altogether, our results suggest a dual role of Bnl/FGF signaling in tracheoblasts, inducing both proliferation and endoreplication, depending on the presence or absence of the transcription factor Ct, respectively.

The regulation of cell size and branch complexity in the terminal cells of the Drosophila tracheal system

The terminal cells of the larval Drosophila tracheal system extend dozens of branched cellular processes, most of which become hollow intracellular tubes that support gas exchange with internal tissues. Previously, we undertook a forward genetic mosaic screen to uncover the pathways regulating terminal cell size, morphogenesis, and the generation and maintenance of new intracellular tubes. Our initial work identified several mutations affecting terminal cell size and branch number, and suggested that branch complexity and cell size are typically coupled but could be genetically separated. To deepen our understanding of these processes, we have further characterized and determined the molecular identities of mutations in the genes sprout, denuded and asthmatic, that had been implicated in our initial screen. Here we reveal the molecular identity of these genes and describe their function in the context of the TOR and Hippo pathways, which are widely appreciated to be key regulators of cell and organ size.

Development and Function of the Drosophila Tracheal System

The tracheal system of insects is a network of epithelial tubules that functions as a respiratory organ to supply oxygen to various target organs. Target-derived signaling inputs regulate stereotyped modes of cell specification, branching morphogenesis, and collective cell migration in the embryonic stage. In the postembryonic stages, the same set of signaling pathways controls highly plastic regulation of size increase and pattern elaboration during larval stages, and cell proliferation and reprograming during metamorphosis. Tracheal tube morphogenesis is also regulated by physicochemical interaction of the cell and apical extracellular matrix to regulate optimal geometry suitable for air flow. The trachea system senses both the external oxygen level and the metabolic activity of internal organs, and helps organismal adaptation to changes in environmental oxygen level. Cellular and molecular mechanisms underlying the high plasticity of tracheal development and physiology uncovered through research on Drosophila are discussed.

Characterization of a novel Drosophila melanogaster cis-regulatory module that drives gene expression to the larval tracheal system and adult thoracic musculature

In a previous bioinformatics analysis we identified 10 conserved Drosophila melanogaster sequences that reside upstream from protein coding genes (CGs). Here we characterize one of these genomic regions, which constitutes a Drosophila melanogaster cis-regulatory module (CRM) that we denominate TT-CRM. The TT-CRM is 646 bp long and is located in one of the introns of CG32239 and resides about 3,500 bp upstream of CG13711 and about 620 bp upstream of CG12493. Analysis of 646 bp-lacZ lines revealed that TT-CRM drives gene expression not only to the larval, prepupal, and pupal tracheal system but also to the adult dorsal longitudinal muscles. The patterns of mRNA expression of the transgene and of the CGs that lie in the vicinity of TT-CRM were investigated both in dissected trachea and in adult thoraces. Through RT-qPCR we observed that in the tracheal system the pattern of expression of 646 bp-lacZ is similar to the pattern of expression of CG32239 and CG13711, whereas in the thoracic muscles 646 bp-lacZ expression accompanies the expression of CG12493. Together, these results suggest new functions for two previously characterized D. melanogaster genes and also contribute to the initial characterization of a novel CRM that drives a dynamic pattern of expression throughout development.

Negative regulation of G2-M by ATR (mei-41)/Chk1(Grapes) facilitates tracheoblast growth and tracheal hypertrophy in Drosophila

Imaginal progenitors in Drosophila are known to arrest in G2 during larval stages and proliferate thereafter. Here we investigate the mechanism and implications of G2 arrest in progenitors of the adult thoracic tracheal epithelium (tracheoblasts). We report that tracheoblasts pause in G2 for ~48–56 h and grow in size over this period. Surprisingly, tracheoblasts arrested in G2 express drivers of G2-M like Cdc25/String (Stg). We find that mechanisms that prevent G2-M are also in place in this interval. Tracheoblasts activate Checkpoint Kinase 1/Grapes (Chk1/Grp) in an ATR/mei-41-dependent manner. Loss of ATR/Chk1 led to precocious mitotic entry ~24–32 h earlier. These divisions were apparently normal as there was no evidence of increased DNA damage or cell death. However, induction of precocious mitoses impaired growth of tracheoblasts and the tracheae they comprise. We propose that ATR/Chk1 negatively regulate G2-M in developing tracheoblasts and that G2 arrest facilitates cellular and hypertrophic organ growth.

Every organism begins as a single cell. That cell, and all the other cells it generates over time, need to divide at the right time and in the right place to develop into an adult. As they do so, they pass through the stages of the cell cycle. As cells prepare to divide they enter into the first growth phase, G1, ramping up their metabolic activity. They then enter S phase, duplicate their DNA, and subsequently a second growth phase G2. Finally, during the mitotic phase, the chromosome separate and cells undergo cytokinesis to form new cells.

Dividing cells can pause at certain stages of the cell cycle to assess whether the conditions are suitable to proceed. The length of the pause depends on the stage of development and the cell type. Signals around the cell provide the cues that it needs to make the decision.

The fruit fly Drosophila melanogaster, for example, undergoes metamorphosis during development, meaning it transforms from a larva into an adult. The larva contains small patches of ‘progenitor’ cells that form the adult tissue. These remain paused for various intervals during larval life and restart their cell cycle as the animal develops. A key challenge in biology is to understand how these progenitors pause and what makes them start dividing again. Here, Kizhedathu, Bagul and Guha uncover a new mechanism that pauses the cell cycle in developing animal cells.

Progenitors of the respiratory system in the adult fruit fly pause at the G2 stage of the cell cycle during larval life. Some of these progenitors, from a part of the larva called the dorsal trunk, go on to form the structures of the adult respiratory system. By counting the cells and tracking their dynamics with fluorescent labels, Kizhedathu et al. revealed that the progenitor cells pause for between 48 and to 56 hours. Previous research suggested that this pause happens because the cells lack a protein essential for mitosis called Cdc25/String. However, these progenitors were producing Cdc25/String. They stopped dividing because they also made another protein, known as Checkpoint Kinase 1/Grapes (Chk1/Grp).

Chk1 is known to add a chemical modification to Cdc25, which dampens its activity and stops the cell cycle from progressing. This is likely what allow the flies to co-ordinate their development and give the cells more time to grow. When Chk1 was experimentally removed, it reactivated the paused cells sooner, resulting in smaller cells and a smaller respiratory organ.

This work extends our understanding of stem cell dynamics and growth during development. Previous work has shown that cells that give rise to muscles and the neural tube (the precursor of the central nervous system) also pause their cell cycle in G2. Understanding more about how this happens could open new avenues for research into developmental disease.

Tumorigenic Properties of Drosophila Epithelial Cells Mutant for lethal giant larvae

BACKGROUND

Mutations in Drosophila tumor suppressor genes (TSGs) lead to the formation of invasive tumors in the brain and imaginal discs.

RESULTS

Here we studied the tumorigenic properties of imaginal discs mutant for the TSG gene lethal giant larvae (lgl). lgl mutant cells display the characteristic features of mammalian tumor cells: they can proliferate indefinitely, induce additional tracheogenesis (an insect counterpart of vasculogenesis) and invade neighboring tissues. Lgl mutant tissues exhibit high apoptotic levels, which lead to the activation of the Jun-N-Terminal Kinase (JNK) pathway. We propose that JNK is a key factor in the acquisition of these tumorigenic properties; it promotes cell proliferation and induces high levels of Mmp1 and confers tumor cells capacity to invade wild-type tissue. Noteworthy, lgl RNAi-mediated down-regulation does not produce similar transformations in the central nervous system (CNS), thereby indicating a fundamental difference between the cells of developing imaginal discs and those of differentiated organs. We discuss these results in the light of the "single big-hit origin" of some human pediatric or developmental cancers.

CONCLUSIONS

Down-regulation of lgl in imaginal discs is sufficient to enhance tracheogenesis and to promote invasion and colonization of other larval structures including the CNS. Developmental Dynamics 245:834-843, 2016. © 2016 Wiley Periodicals, Inc.

Genetic basis for the evolution of organ morphogenesis: the case of spalt and cut in the development of insect trachea

It is not clear how simple genetic changes can account for the coordinated variations that give rise to modified functional organs. Here, we addressed this issue by analysing the expression and function of regulatory genes in the developing tracheal systems of two insect species. The larval tracheal system of Drosophila can be distinguished from the less derived tracheal system of the beetle Tribolium by two main features. First, Tribolium has lateral spiracles connecting the trachea to the exterior in each segment, while Drosophila has only one pair of posterior spiracles. Second, Drosophila, but not Tribolium, has two prominent longitudinal branches that distribute air from the posterior spiracles. Both innovations, while considered different structures, are functionally dependent on each other and linked to habitat occupancy. We show that changes in the domains of spalt and cut expression in the embryo are associated with the acquisition of each structure. Moreover, we show that these two genetic modifications are connected both functionally and genetically, thus providing an evolutionary scenario by which a genetic event contributes to the joint evolution of functionally inter-related structures.

Snoo and Dpp Act as Spatial and Temporal Regulators Respectively of Adult Progenitor Cells in the Drosophila Trachea

Clusters of differentiated cells contributing to organ structures retain the potential to re-enter the cell cycle and replace cells lost during development or upon damage. To do so, they must be designated spatially and respond to proper activation cues. Here we show that in the case of Drosophila differentiated larval tracheal cells, progenitor potential is conferred by the spatially restricted activity of the Snoo transcription cofactor. Furthermore, Dpp signalling regulated by endocrine hormonal cues provides the temporal trigger for their activation. Finally, we elucidate the genetic network elicited by Snoo and Dpp activity. These results illustrate a regulatory mechanism that translates intrinsic potential and extrinsic cues into the facultative stem cell features of differentiated progenitors.

An important feature of organs is their ability to maintain their structure and function in spite of natural or accidental cell loss. This capacity is often sustained by so-called stem cells, which are able to provide new cells of the different types in the organ. In addition, some specialized cells, known as facultative stem cells, also retain the ability to re-enter the cell cycle and replace lost tissue. This process has to be very precisely regulated to provide for the maintenance of the tissues and organs while preventing uncontrolled cellular growth. We have analysed this mechanism in the Drosophila trachea; there, a group of Differentiated Adult Progenitor cells (or DAP cells) share the features of facultative stem cells as they remain quiescent during larval growth, reactivate their proliferation at the last larval stage and give rise to the different cell types of the adult tracheal network during metamorphosis. The DAP cells, conversely to the majority of the larval cells, do not enter endocycle and by doing so they acquire the features of adult progenitor cells. In this paper we identify the regulatory mechanism that integrates spatial and temporal cues to precisely activate the tracheal adult progenitor program.

Effects of human food grade titanium dioxide nanoparticle dietary exposure on Drosophila melanogaster survival, fecundity, pupation and expression of antioxidant genes

The fruitfly, Drosophila melanogaster was exposed to the human food grade of E171 titanium dioxide (TiO2). This is a special grade of TiO2 which is frequently omitted in nanotoxicology studies dealing with TiO2, yet it is the most relevant grade regarding oral exposure of humans. D. melanogaster larvae were exposed to 0.002 mg mL(-1), 0.02 mg mL(-1), 0.2 mg mL(-1), and 2 mg mL(-1) of TiO2 in feeding medium, and the survival, fecundity, pupation time, and expression of genes involved in oxidative stress response were monitored. TiO2 did not affect survival but significantly increased time to pupation (p < 0.001). Fecundity of D. melanogaster was unaffected by the treatment. Expression of the gene for catalase was markedly downregulated by the treatment, while the effect on the downregulation of superoxide dismutase 2 was less pronounced. After four days of dietary exposure TiO2 was present in a significant amount in larvae, but was not transferred to adults during metamorphosis. Two individuals with aberrant phenotype similar to previously described gold nanoparticles induced mutant phenotypes were detected in the group exposed to TiO2. In general, TiO2 showed little toxicity toward D. melanogaster at concentrations relevant to oral exposure of humans.

Drosophila Lung Cancer Models Identify Trametinib plus Statin as Candidate Therapeutic

We have developed a Drosophila lung cancer model by targeting Ras1(G12V)--alone or in combination with PTEN knockdown--to the Drosophila tracheal system. This led to overproliferation of tracheal tissue, formation of tumor-like growths, and animal lethality. Screening a library of FDA-approved drugs identified several that improved overall animal survival. We explored two hits: the MEK inhibitor trametinib and the HMG-CoA reductase inhibitor fluvastatin. Oral administration of these drugs inhibited Ras and PI3K pathway activity, respectively; in addition, fluvastatin inhibited protein prenylation downstream of HMG-CoA reductase to promote survival. Combining drugs led to synergistic suppression of tumor formation and rescue lethality; similar synergy was observed in human A549 lung adenocarcinoma cells. Notably, fluvastatin acted both within transformed cells and also to reduce whole-body trametinib toxicity in flies. Our work supports and provides further context for exploring the potential of combining statins with MAPK inhibitors such as trametinib to improve overall therapeutic index.

Multipotent versus differentiated cell fate selection in the developing Drosophila airways

Developmental potentials of cells are tightly controlled at multiple levels. The embryonic Drosophila airway tree is roughly subdivided into two types of cells with distinct developmental potentials: a proximally located group of multipotent adult precursor cells (P-fate) and a distally located population of more differentiated cells (D-fate). We show that the GATA-family transcription factor (TF) Grain promotes the P-fate and the POU-homeobox TF Ventral veinless (Vvl/Drifter/U-turned) stimulates the D-fate. Hedgehog and receptor tyrosine kinase (RTK) signaling cooperate with Vvl to drive the D-fate at the expense of the P-fate while negative regulators of either of these signaling pathways ensure P-fate specification. Local concentrations of Decapentaplegic/BMP, Wingless/Wnt, and Hedgehog signals differentially regulate the expression of D-factors and P-factors to transform an equipotent primordial field into a concentric pattern of radially different morphogenetic potentials, which gradually gives rise to the distal-proximal organization of distinct cell types in the mature airway.

DOI:http://dx.doi.org/10.7554/eLife.09646.001

Many organs are composed of tubes of different sizes, shapes and patterns that transport vital substances from one site to another. In the fruit fly species Drosophila melanogaster, oxygen is transported by a tubular network, which divides into finer tubes that allow the oxygen to reach every part of the body.

Different parts of the fruit fly’s airways develop from different groups of tracheal precursor cells. P-fate cells form the most 'proximal' tubes (which are found next to the outer layer of the fly). These cells are 'multipotent' stem cells, and have the ability to specialize into many different types of cells during metamorphosis. The more 'distal' branches that emerge from the proximal tubes develop from D-fate cells. These are cells that generally acquire a narrower range of cell identities.

By performing a genetic analysis of fruit fly embryos, Matsuda et al. have now identified several proteins and signaling molecules that control whether tracheal precursor cells become D-fate or P-fate cells. For example, several signaling pathways work with a protein called Ventral veinless to cause D-fate cells to develop instead of P-fate cells. However, molecules that prevent signaling occurring via these pathways help P-fate cells to form. Different amounts of the molecules that either promote or hinder these signaling processes are present in different parts of the fly embryo; this helps the airways of the fly to develop in the correct pattern.

This work provides a comprehensive view of how cell types with different developmental potentials are positioned in a complex tubular network. This sets a basis for future studies addressing how the respiratory organs – and indeed the entire organism – are sustained.

DOI:http://dx.doi.org/10.7554/eLife.09646.002

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

A cellular process that includes asymmetric cytokinesis remodels the dorsal tracheal branches in Drosophila larvae

Tubular networks are central to the structure and function of many organs, such as the vertebrate lungs or the Drosophila tracheal system. Their component epithelial cells are able to proliferate and to undergo complex morphogenetic movements, while maintaining their barrier function. Little is known about the details of the mitotic process in tubular epithelia. Our study presents a comprehensive model of cellular remodeling and proliferation in the dorsal branches of third-instar Drosophila larvae. Through a combination of immunostaining and novel live imaging techniques, we identify the key steps in the transition from a unicellular to a multicellular tube. Junctional remodeling precedes mitosis and, as the cells divide, new junctions are formed through several variations of what we refer to as 'asymmetric cytokinesis'. Depending on the spacing of cells along the dorsal branch, mitosis can occur either before or after the transition to a multicellular tube. In both instances, cell separation is accomplished through asymmetric cytokinesis, a process that is initiated by the ingression of the cytokinetic ring. Unequal cell compartments are a possible but rare outcome of completing mitosis through this mechanism. We also found that the Dpp signaling pathway is required but not sufficient for cell division in the dorsal branches.

Compensatory branching morphogenesis of stalk cells in the Drosophila trachea

Tubes are essential for nutrient transport and gas exchange in multicellular eukaryotes, but how connections between different tube types are maintained over time is unknown. In the Drosophila tracheal system, mutations in oak gall (okg) and conjoined (cnj) confer identical defects, including late onset blockage near the terminal cell-stalk cell junction and the ectopic extension of autocellular, seamed tubes into the terminal cell. We determined that okg and cnj encode the E and G subunits of the vacuolar ATPase (vATPase) and showed that both the V0 and V1 domains are required for terminal cell morphogenesis. Remarkably, the ectopic seamed tubes running along vATPase-deficient terminal cells belonged to the neighboring stalk cells. All vATPase-deficient tracheal cells had reduced apical domains and terminal cells displayed mislocalized apical proteins. Consistent with recent reports that the mTOR and vATPase pathways intersect, we found that mTOR pathway mutants phenocopied okg and cnj. Furthermore, terminal cells depleted for the apical determinants Par6 or aPKC had identical ectopic seamed tube defects. We thus identify a novel mechanism of compensatory branching in which stalk cells extend autocellular tubes into neighboring terminal cells with undersized apical domains. This compensatory branching also occurs in response to injury, with damaged terminal cells being rapidly invaded by their stalk cell neighbor.

Subcellular trafficking of FGF controls tracheal invasion of Drosophila flight muscle

To meet the extreme oxygen demand of insect flight muscle, tracheal (respiratory) tubes ramify not only on its surface, as in other tissues, but also within T-tubules and ultimately surrounding every mitochondrion. Although this remarkable physiological specialization has long been recognized, its cellular and molecular basis is unknown. Here, we show that Drosophila tracheoles invade flight muscle T-tubules through transient surface openings. Like other tracheal branching events, invasion requires the Branchless FGF pathway. However, localization of the FGF chemoattractant changes from all muscle membranes to T-tubules as invasion begins. Core regulators of epithelial basolateral membrane identity localize to T-tubules, and knockdown of AP-1γ, required for basolateral trafficking, redirects FGF from T-tubules to surface, increasing tracheal surface ramification and preventing invasion. We propose that tracheal invasion is controlled by an AP-1-dependent switch in FGF trafficking. Thus, subcellular targeting of a chemoattractant can direct outgrowth to specific domains, including inside the cell.

Transcriptional regionalization of the fruit fly's airway epithelium

Although airway epithelia are primarily devoted to gas exchange, they have to fulfil a number of different tasks including organ maintenance and the epithelial immune response to fight airborne pathogens. These different tasks are at least partially accomplished by specialized cell types in the epithelium. In addition, a proximal to distal gradient mirroring the transition from airflow conduction to real gas exchange, is also operative. We analysed the airway system of larval Drosophila melanogaster with respect to region-specific expression in the proximal to distal axis. The larval airway system is made of epithelial cells only. We found differential expression between major trunks of the airways and more distal ones comprising primary, secondary and terminal ones. A more detailed analysis was performed using DNA-microarray analysis to identify cohorts of genes that are either predominantly expressed in the dorsal trunks or in the primary/secondary/terminal branches of the airways. Among these differentially expressed genes are especially those involved in signal transduction. Wnt-signalling associated genes for example are predominantly found in secondary/terminal airways. In addition, some G-protein coupled receptors are differentially expressed between both regions of the airways, exemplified by those activated by octopamine or tyramine, the invertebrate counterparts of epinephrine and norepinephrine. Whereas the OAMB is predominantly found in terminal airway regions, the oct3βR has higher expression levels in dorsal trunks. In addition, we observed a significant association of both, genes predominantly expressed in dorsal trunks or in primary to terminal branches branches with those regulated by hypoxia. Taken together, this observed differential expression is indicative for a proximal to distal transcriptional regionalization presumably reflecting functional differences in these parts of the fly’s airway system.

IV. Tools and methods for studying cell migration and cell rearrangement in tissue and organ development

A vast diversity of biological systems, ranging from prokaryotes to multicellular organisms, show cell migration behavior. Many of the basic cellular and molecular concepts in cell migration apply to diverse model organisms. Drosophila, with its vast repertoire of tools for imaging and for manipulation, is one of the favorite organisms to study cell migration. Moreover, distinct Drosophila tissues and organs offer diverse cell migration models that are amenable to live imaging and genetic manipulations. In this review, we will provide an overview of the fruit fly toolbox that is of particular interest for the analysis of cell migration. We provide examples to highlight how those tools were used in diverse migration systems, with an emphasis on tracheal morphogenesis, a process that combines morphogenesis with cell migration.

Progenitor outgrowth from the niche in Drosophila trachea is guided by FGF from decaying branches

Although there has been progress identifying adult stem and progenitor cells and the signals that control their proliferation and differentiation, little is known about the substrates and signals that guide them out of their niche. By examining Drosophila tracheal outgrowth during metamorphosis, we show that progenitors follow a stereotyped path out of the niche, tracking along a subset of tracheal branches destined for destruction. The embryonic tracheal inducer branchless FGF (fibroblast growth factor) is expressed dynamically just ahead of progenitor outgrowth in decaying branches. Knockdown of branchless abrogates progenitor outgrowth, whereas misexpression redirects it. Thus, reactivation of an embryonic tracheal inducer in decaying branches directs outgrowth of progenitors that replace them. This explains how the structure of a newly generated tissue is coordinated with that of the old.

FGF /FGFR signal induces trachea extension in the drosophila visual system

The Drosophila compound eye is a large sensory organ that places a high demand on oxygen supplied by the tracheal system. Although the development and function of the Drosophila visual system has been extensively studied, the development and contribution of its tracheal system has not been systematically examined. To address this issue, we studied the tracheal patterns and developmental process in the Drosophila visual system. We found that the retinal tracheae are derived from air sacs in the head, and the ingrowth of retinal trachea begin at mid-pupal stage. The tracheal development has three stages. First, the air sacs form near the optic lobe in 42-47% of pupal development (pd). Second, in 47-52% pd, air sacs extend branches along the base of the retina following a posterior-to-anterior direction and further form the tracheal network under the fenestrated membrane (TNUFM). Third, the TNUFM extend fine branches into the retina following a proximal-to-distal direction after 60% pd. Furthermore, we found that the trachea extension in both retina and TNUFM are dependent on the FGF(Bnl)/FGFR(Btl) signaling. Our results also provided strong evidence that the photoreceptors are the source of the Bnl ligand to guide the trachea ingrowth. Our work is the first systematic study of the tracheal development in the visual system, and also the first study demonstrating the interactions of two well-studied systems: the eye and trachea.

Neurodegeneration in drop-dead mutant drosophila melanogaster is associated with the respiratory system but not with Hypoxia

Mutations in the gene drop-dead (drd) cause diverse phenotypes in adult Drosophila melanogaster including early lethality, neurodegeneration, tracheal defects, gut dysfunction, reduced body mass, and female sterility. Despite the identification of the drd gene itself, the causes of early lethality and neurodegeneration in the mutant flies remain unknown. To determine the pattern of drd expression associated with the neurodegenerative phenotype, knockdown of drd with various Gal4 drivers was performed. Early adult lethality and neurodegeneration were observed upon knockdown of drd in the tracheal system with two independent insertions of the breathless-Gal4 driver and upon knockdown in the tracheal system and elsewhere with the DJ717-Gal4 driver. Surprisingly, rescue of drd expression exclusively in the tracheae in otherwise mutant flies rescued the neurodegenerative phenotype but not adult lethality. Gut dysfunction, as measured by defecation rate, was not rescued in these flies, and gut function appeared normal upon tracheal-specific knockdown of drd. Finally, the hypothesis that tracheal dysfunction in drd mutants results in hypoxia was tested. Hypoxia-sensitive reporter transgenes (LDH-Gal4 and LDH-LacZ) were placed on a drd mutant background, but enhanced expression of these reporters was not observed. In addition, manipulation of drd expression in the tracheae did not affect expression of the hypoxia-induced genes LDH, tango, and similar. Overall, these results indicate that there are at least two causes of adult lethality in drd mutants, that gut dysfunction and neurodegeneration are independent phenotypes, and that neurodegeneration is associated with tracheal expression of drd but not with hypoxia.

Receptor tyrosine kinases in Drosophila development

Tyrosine phosphorylation plays a significant role in a wide range of cellular processes. The Drosophila genome encodes more than 20 receptor tyrosine kinases and extensive studies in the past 20 years have illustrated their diverse roles and complex signaling mechanisms. Although some receptor tyrosine kinases have highly specific functions, others strikingly are used in rather ubiquitous manners. Receptor tyrosine kinases regulate a broad expanse of processes, ranging from cell survival and proliferation to differentiation and patterning. Remarkably, different receptor tyrosine kinases share many of the same effectors and their hierarchical organization is retained in disparate biological contexts. In this comprehensive review, we summarize what is known regarding each receptor tyrosine kinase during Drosophila development. Astonishingly, very little is known for approximately half of all Drosophila receptor tyrosine kinases.

The homeobox transcription factor cut coordinates patterning and growth during Drosophila airway remodeling

A fundamental question in developmental biology is how tissue growth and patterning are coordinately regulated to generate complex organs with characteristic shapes and sizes. We showed that in the developing primordium that produces the Drosophila adult trachea, the homeobox transcription factor Cut regulates both growth and patterning, and its effects depend on its abundance. Quantification of the abundance of Cut in the developing airway progenitors during late larval stage 3 revealed that the cells of the developing trachea had different amounts of Cut, with the most proliferative region having an intermediate amount of Cut and the region lacking Cut exhibiting differentiation. By manipulating Cut abundance, we showed that Cut functioned in different regions to regulate proliferation or patterning. Transcriptional profiling of progenitor populations with different amounts of Cut revealed the Wingless (known as Wnt in vertebrates) and Notch signaling pathways as positive and negative regulators of cut expression, respectively. Furthermore, we identified the gene encoding the receptor Breathless (Btl, known as fibroblast growth factor receptor in vertebrates) as a transcriptional target of Cut. Cut inhibited btl expression and tracheal differentiation to maintain the developing airway cells in a progenitor state. Thus, Cut functions in the integration of patterning and growth in a developing epithelial tissue.

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.

Branching morphogenesis: from cells to organs and back

Many animal organs, such as the lung, the kidney, the mammary gland, and the vasculature, consist of branched tubular structures that arise through a process known as "branching morphogenesis" that results from the remodeling of epithelial or endothelial sheaths into multicellular tubular networks. In recent years, the combination of molecular biology, forward and reverse genetic approaches, and their complementation by live imaging has started to unravel rules and mechanisms controlling branching processes in animals. Common patterns of branch formation spanning diverse model systems are beginning to emerge that might reflect unifying principles of tubular organ formation.

Comparative molecular developmental aspects of the mammalian- and the avian lungs, and the insectan tracheal system by branching morphogenesis: recent advances and future directions

Gas exchangers fundamentally form by branching morphogenesis (BM), a mechanistically profoundly complex process which derives from coherent expression and regulation of multiple genes that direct cell-to-cell interactions, differentiation, and movements by signaling of various molecular morphogenetic cues at specific times and particular places in the developing organ. Coordinated expression of growth-instructing factors determines sizes and sites where bifurcation occurs, by how much a part elongates before it divides, and the angle at which branching occurs. BM is essentially induced by dualities of factors where through feedback- or feed forward loops agonists/antagonists are activated or repressed. The intricate transactions between the development orchestrating molecular factors determine the ultimate phenotype. From the primeval time when the transformation of unicellular organisms to multicellular ones occurred by systematic accretion of cells, BM has been perpetually conserved. Canonical signalling, transcriptional pathways, and other instructive molecular factors are commonly employed within and across species, tissues, and stages of development. While much still remain to be elucidated and some of what has been reported corroborated and reconciled with rest of existing data, notable progress has in recent times been made in understanding the mechanism of BM. By identifying and characterizing the morphogenetic drivers, and markers and their regulatory dynamics, the elemental underpinnings of BM have been more precisely explained. Broadening these insights will allow more effective diagnostic and therapeutic interventions of developmental abnormalities and pathologies in pre- and postnatal lungs. Conservation of the molecular factors which are involved in the development of the lung (and other branched organs) is a classic example of nature's astuteness in economically utilizing finite resources. Once purposefully formed, well-tested and tried ways and means are adopted, preserved, and widely used to engineer the most optimal phenotypes. The material and time costs of developing utterly new instruments and routines with every drastic biological change (e.g. adaptation and speciation) are circumvented. This should assure the best possible structures and therefore functions, ensuring survival and evolutionary success.

Drosophila models of epithelial stem cells and their niches

Epithelial stem cells are regulated through a complex interplay of signals from diffusible ligands, cellular interactions, and attachment to the extracellular matrix. The development of Drosophila models of epithelial stem cells and their associated niche has made it possible to dissect the contribution of each of these factors in vivo, during both basal homeostasis and in response to acute damage such as infection. Studies of Drosophila epithelial stem cells have also provided insight into the mechanisms by which a healthy population of stem cells are maintained throughout adulthood by demonstrating, for example, that stem cells have a finite lifespan and may be displaced by replacement cells competing for niche occupancy. Here, we summarize the literature on each of the known Drosophila epithelial stem cells, with a focus on the two most well-characterized types, the follicle stem cells (FSCs) in the ovary and the intestinal stem cells (ISCs) in the posterior midgut. Several themes have emerged from these studies, which suggest that there may be a common set of features among niches in a variety of epithelia. For example, unlike the simpler Drosophila germline stem cell niches, both the FSC and ISC niches produce multiple, partially redundant, niche signals, some of which activate pathways such as Wnt/Wingless, Hedgehog, and epidermal growth factor (EGF) that also regulate mammalian epithelial tissue renewal. Further study into these relatively new stem cell models will be of use in understanding both the specifics of epithelial regeneration and the diversity of mechanisms that regulate adult stem cells in general.

The peripheral nervous system supports blood cell homing and survival in the Drosophila larva

Interactions of hematopoietic cells with their microenvironment control blood cell colonization, homing and hematopoiesis. Here, we introduce larval hematopoiesis as the first Drosophila model for hematopoietic colonization and the role of the peripheral nervous system (PNS) as a microenvironment in hematopoiesis. The Drosophila larval hematopoietic system is founded by differentiated hemocytes of the embryo, which colonize segmentally repeated epidermal-muscular pockets and proliferate in these locations. Importantly, we show that these resident hemocytes tightly colocalize with peripheral neurons and we demonstrate that larval hemocytes depend on the PNS as an attractive and trophic microenvironment. atonal (ato) mutant or genetically ablated larvae, which are deficient for subsets of peripheral neurons, show a progressive apoptotic decline in hemocytes and an incomplete resident hemocyte pattern, whereas supernumerary peripheral neurons induced by ectopic expression of the proneural gene scute (sc) misdirect hemocytes to these ectopic locations. This PNS-hematopoietic connection in Drosophila parallels the emerging role of the PNS in hematopoiesis and immune functions in vertebrates, and provides the basis for the systematic genetic dissection of the PNS-hematopoietic axis in the future.

Notch and Ras promote sequential steps of excretory tube development in C. elegans

Receptor tyrosine kinases and Notch are crucial for tube formation and branching morphogenesis in many systems, but the specific cellular processes that require signaling are poorly understood. Here we describe sequential roles for Notch and Epidermal growth factor (EGF)-Ras-ERK signaling in the development of epithelial tube cells in the C. elegans excretory (renal-like) organ. This simple organ consists of three tandemly connected unicellular tubes: the excretory canal cell, duct and G1 pore. lin-12 and glp-1/Notch are required to generate the canal cell, which is a source of LIN-3/EGF ligand and physically attaches to the duct during de novo epithelialization and tubulogenesis. Canal cell asymmetry and let-60/Ras signaling influence which of two equivalent precursors will attach to the canal cell. Ras then specifies duct identity, inducing auto-fusion and a permanent epithelial character; the remaining precursor becomes the G1 pore, which eventually loses epithelial character and withdraws from the organ to become a neuroblast. Ras continues to promote subsequent aspects of duct morphogenesis and differentiation, and acts primarily through Raf-ERK and the transcriptional effectors LIN-1/Ets and EOR-1. These results reveal multiple genetically separable roles for Ras signaling in tube development, as well as similarities to Ras-mediated control of branching morphogenesis in more complex organs, including the mammalian kidney. The relative simplicity of the excretory system makes it an attractive model for addressing basic questions about how cells gain or lose epithelial character and organize into tubular networks.

Embryonic multipotent progenitors remodel the Drosophila airways during metamorphosis

Adult structures in holometabolous insects such as Drosophila are generated by groups of imaginal cells dedicated to the formation of different organs. Imaginal cells are specified in the embryo and remain quiescent until the larval stages, when they proliferate and differentiate to form organs. The Drosophila tracheal system is extensively remodeled during metamorphosis by a small number of airway progenitors. Among these, the spiracular branch tracheoblasts are responsible for the generation of the pupal and adult abdominal airways. To understand the coordination of proliferation and differentiation during organogenesis of tubular organs, we analyzed the remodeling of Drosophila airways during metamorphosis. We show that the embryonic spiracular branch tracheoblasts are multipotent cells that express the homeobox transcription factor Cut, which is necessary for their survival and normal development. They give rise to three distinct cell populations at the end of larval development, which generate the adult tracheal tubes, the spiracle and the epidermis surrounding the spiracle. Our study establishes the series of events that lead to the formation of an adult tubular structure in Drosophila.

Lifespan and oxidative stress show a non-linear response to atmospheric oxygen in Drosophila

Oxygen provides the substrate for most ATP production, but also serves as a source of reactive oxygen species (ROS), which can induce cumulative macromolecular oxidative damage and cause aging. Pure oxygen atmospheres (100 kPa) are known to strongly reduce invertebrate lifespan and induce aging-related physiological changes. However, the nature of the relationship between atmospheric oxygen, oxidative stress, and lifespan across a range of oxygen levels is poorly known. Developmental responses are likely to play a strong role, as prior research has shown strong effects of rearing oxygen level on growth, size and respiratory system morphology. In this study, we examined (1) the effect of oxygen on adult longevity and (2) the effect of the oxygen concentration experienced by larvae on adult lifespan by rearing Drosophila melanogaster in three oxygen atmospheres throughout larval development (10, 21 and 40 kPa), then measuring the lifespan of adults in five oxygen tensions (2, 10, 21, 40, 100 kPa). We also assessed the rate of protein carbonyl production for flies kept at 2, 10, 21, 40 and 100 kPa as adults (all larvae reared in normoxia). The rearing of juveniles in varying oxygen treatments affected lifespan in a complex manner, and the effect of different oxygen tensions on adult lifespan was non-linear, with reduced longevity and heightened oxidative stress at extreme high and low atmospheric oxygen levels. Moderate hypoxia (10 kPa) extended maximum, but not mean lifespan.

Tracheal remodelling in response to hypoxia

The insect tracheal system is a continuous tubular network that ramifies into progressively thinner branches to provide air directly to every organ and tissue throughout the body. During embryogenesis the basic architecture of the tracheal system develops in a stereotypical and genetically controlled manner. Later, in larval stages, the tracheal system becomes plastic, and adapts to particular oxygen needs of the different tissues of the body. Oxygen sensing is mediated by specific prolyl-4-hydroxylases that regulate protein stability of the alpha subunit of oxygen-responsive transcription factors from the HIF family. Tracheal cells are exquisitely sensitive to oxygen levels, modulating the expression of hypoxia-inducible proteins that mediate sprouting of tracheal branches in direction to oxygen-deprived tissues.

The Hrs/Stam complex acts as a positive and negative regulator of RTK signaling during Drosophila development

BACKGROUND

Endocytosis is a key regulatory step of diverse signalling pathways, including receptor tyrosine kinase (RTK) signalling. Hrs and Stam constitute the ESCRT-0 complex that controls the initial selection of ubiquitinated proteins, which will subsequently be degraded in lysosomes. It has been well established ex vivo and during Drosophila embryogenesis that Hrs promotes EGFR down regulation. We have recently isolated the first mutations of stam in flies and shown that Stam is required for air sac morphogenesis, a larval respiratory structure whose formation critically depends on finely tuned levels of FGFR activity. This suggest that Stam, putatively within the ESCRT-0 complex, modulates FGF signalling, a possibility that has not been examined in Drosophila yet.

PRINCIPAL FINDINGS

Here, we assessed the role of the Hrs/Stam complex in the regulation of signalling activity during Drosophila development. We show that stam and hrs are required for efficient FGFR signalling in the tracheal system, both during cell migration in the air sac primordium and during the formation of fine cytoplasmic extensions in terminal cells. We find that stam and hrs mutant cells display altered FGFR/Btl localisation, likely contributing to impaired signalling levels. Electron microscopy analyses indicate that endosome maturation is impaired at distinct steps by hrs and stam mutations. These somewhat unexpected results prompted us to further explore the function of stam and hrs in EGFR signalling. We show that while stam and hrs together downregulate EGFR signalling in the embryo, they are required for full activation of EGFR signalling during wing development.

CONCLUSIONS/SIGNIFICANCE

Our study shows that the ESCRT-0 complex differentially regulates RTK signalling, either positively or negatively depending on tissues and developmental stages, further highlighting the importance of endocytosis in modulating signalling pathways during development.

Gene switching at Xenopus laevis metamorphosis

During the climax of amphibian metamorphosis many tadpole organs remodel. The different remodeling strategies are controlled by thyroid hormone (TH). The liver, skin, and tail fibroblasts shut off tadpole genes and activate frog genes in the same cell without DNA replication. We refer to this as "gene switching". In contrast, the exocrine pancreas and the intestinal epithelium dedifferentiate to a progenitor state and then redifferentiate to the adult cell type. Tadpole and adult globin are not present in the same cell. Switching from red cells containing tadpole-specific globin to those with frog globin in the liver occurs at a progenitor cell stage of development and is preceded by DNA replication. Red cell switching is the only one of these remodeling strategies that resembles a stem cell mechanism.

A luminal epithelial stem cell that is a cell of origin for prostate cancer

In epithelial tissues, the lineage relationship between normal progenitor cells and cell type(s) of origin for cancer has been poorly understood. Here we show that a known regulator of prostate epithelial differentiation, the homeobox gene Nkx3.1, marks a stem cell population that functions during prostate regeneration. Genetic lineage-marking demonstrates that rare luminal cells which express Nkx3.1 in the absence of testicular androgens (castration-resistant Nkx3.1-expressing cells, CARNs) are bipotential and can self-renew in vivo, while single-cell transplantation assays show that CARNs can reconstitute prostate ducts in renal grafts. Functional assays of Nkx3.1 mutant mice in serial prostate regeneration assays suggest that Nkx3.1 is required for stem cell maintenance. Finally, targeted deletion of the Pten tumor suppressor gene in CARNs results in rapid formation of carcinoma following androgen-mediated regeneration. These observations indicate that CARNs represent a novel luminal stem cell population that is an efficient target for oncogenic transformation in prostate cancer.

Neuregulin1/ErbB4 signaling induces cardiomyocyte proliferation and repair of heart injury

Many organs rely on undifferentiated stem and progenitor cells for tissue regeneration. Whether differentiated cells themselves can contribute to cell replacement and tissue regeneration is a controversial question. Here, we show that differentiated heart muscle cells, cardiomyocytes, can be induced to proliferate and regenerate. We identify an underlying molecular mechanism for controlling this process that involves the growth factor neuregulin1 (NRG1) and its tyrosine kinase receptor, ErbB4. NRG1 induces mononucleated, but not binucleated, cardiomyocytes to divide. In vivo, genetic inactivation of ErbB4 reduces cardiomyocyte proliferation, whereas increasing ErbB4 expression enhances it. Injecting NRG1 in adult mice induces cardiomyocyte cell-cycle activity and promotes myocardial regeneration, leading to improved function after myocardial infarction. Undifferentiated progenitor cells did not contribute to NRG1-induced cardiomyocyte proliferation. Thus, increasing the activity of the NRG1/ErbB4 signaling pathway may provide a molecular strategy to promote myocardial regeneration.

Patterning mechanisms of branched organs

Branching morphogenesis is one of the earliest events essential for the success of metazoans. By branching out and forming cellular or tissue extensions, cells can maximize their surface area and overcome space constraints posed by organ size. Over the past decade, tremendous progress has been made toward understanding the branching mechanisms of various invertebrate and vertebrate organ systems. Despite their distinct origins, morphologies and functions, different cell and tissue types use a remarkably conserved set of tools to undergo branching morphogenesis. Recent studies have shed important light on the basis of molecular conservation in the formation of branched structures in diverse organs.