Multiple signalling pathways establish cell fate and cell number in Drosophila malpighian tubules

Early patterning followed by tissue growth establishes distal identity in Drosophila Malpighian tubules

Specification and elaboration of proximo-distal (P-D) axes for structures or tissues within a body occurs secondarily from that of the main axes of the body. Our understanding of the mechanism(s) that pattern P-D axes is limited to a few examples such as vertebrate and invertebrate limbs. Drosophila Malpighian/renal tubules (MpTs) are simple epithelial tubules, with a defined P-D axis. How this axis is patterned is not known, and provides an ideal context to understand patterning mechanisms of a secondary axis. Furthermore, epithelial tubules are widespread, and their patterning is not well understood. Here, we describe the mechanism that establishes distal tubule and show this is a radically different mechanism to that patterning the proximal MpT. The distal domain is patterned in two steps: distal identity is specified in a small group of cells very early in MpT development through Wingless/Wnt signalling. Subsequently, this population is expanded by proliferation to generate the distal MpT domain. This mechanism enables distal identity to be established in the tubule in a domain of cells much greater than the effective range of Wingless.

Physiology, Development, and Disease Modeling in the Drosophila Excretory System

The insect excretory system contains two organ systems acting in concert: the Malpighian tubules and the hindgut perform essential roles in excretion and ionic and osmotic homeostasis. For over 350 years, these two organs have fascinated biologists as a model of organ structure and function. As part of a recent surge in interest, research on the Malpighian tubules and hindgut of Drosophila have uncovered important paradigms of organ physiology and development. Further, many human disease processes can be modeled in these organs. Here, focusing on discoveries in the past 10 years, we provide an overview of the anatomy and physiology of the Drosophila excretory system. We describe the major developmental events that build these organs during embryogenesis, remodel them during metamorphosis, and repair them following injury. Finally, we highlight the use of the Malpighian tubules and hindgut as accessible models of human disease biology. The Malpighian tubule is a particularly excellent model to study rapid fluid transport, neuroendocrine control of renal function, and modeling of numerous human renal conditions such as kidney stones, while the hindgut provides an outstanding model for processes such as the role of cell chirality in development, nonstem cell-based injury repair, cancer-promoting processes, and communication between the intestine and nervous system.

Novel roles for GATAe in growth, maintenance and proliferation of cell populations in the Drosophila renal tubule

The GATA family of transcription factors is implicated in numerous developmental and physiological processes in metazoans. In Drosophila melanogaster, five different GATA factor genes (pannier, serpent, grain, GATAd and GATAe) have been reported as essential in the development and identity of multiple tissues, including the midgut, heart and brain. Here, we present a novel role for GATAe in the function and homeostasis of the Drosophila renal (Malpighian) tubule. We demonstrate that reduced levels of GATAe gene expression in tubule principal cells induce uncontrolled cell proliferation, resulting in tumorous growth with associated altered expression of apoptotic and carcinogenic key genes. Furthermore, we uncover the involvement of GATAe in the maintenance of stellate cells and migration of renal and nephritic stem cells into the tubule. Our findings of GATAe as a potential master regulator in the events of growth control and cell survival required for the maintenance of the Drosophila renal tubule could provide new insights into the molecular pathways involved in the formation and maintenance of a functional tissue and kidney disease.

Release and spread of Wingless is required to pattern the proximo-distal axis of Drosophila renal tubules

Wingless/Wnts are signalling molecules, traditionally considered to pattern tissues as long-range morphogens. However, more recently the spread of Wingless was shown to be dispensable in diverse developmental contexts in Drosophila and vertebrates. Here we demonstrate that release and spread of Wingless is required to pattern the proximo-distal (P-D) axis of Drosophila Malpighian tubules. Wingless signalling, emanating from the midgut, directly activates odd skipped expression several cells distant in the proximal tubule. Replacing Wingless with a membrane-tethered version that is unable to diffuse from the Wingless producing cells results in aberrant patterning of the Malpighian tubule P-D axis and development of short, deformed ureters. This work directly demonstrates a patterning role for a released Wingless signal. As well as extending our understanding about the functional modes by which Wnts shape animal development, we anticipate this mechanism to be relevant to patterning epithelial tubes in other organs, such as the vertebrate kidney.

Modeling Renal Disease "On the Fly"

Detoxification is a fundamental function for all living organisms that need to excrete catabolites and toxins to maintain homeostasis. Kidneys are major organs of detoxification that maintain water and electrolyte balance to preserve physiological functions of vertebrates. In insects, the renal function is carried out by Malpighian tubules and nephrocytes. Due to differences in their circulation, the renal systems of mammalians and insects differ in their functional modalities, yet carry out similar biochemical and physiological functions and share extensive genetic and molecular similarities. Evolutionary conservation can be leveraged to model specific aspects of the complex mammalian kidney function in the genetic powerhouse Drosophila melanogaster to study how genes interact in diseased states. Here, we compare the human and Drosophila renal systems and present selected fly disease models.

Transcription factor TFCP2L1 patterns cells in the mouse kidney collecting ducts

Although most nephron segments contain one type of epithelial cell, the collecting ducts consists of at least two: intercalated (IC) and principal (PC) cells, which regulate acid-base and salt-water homeostasis, respectively. In adult kidneys, these cells are organized in rosettes suggesting functional interactions. Genetic studies in mouse revealed that transcription factor Tfcp2l1 coordinates IC and PC development. Tfcp2l1 induces the expression of IC specific genes, including specific H⁺-ATPase subunits and Jag1. Jag1 in turn, initiates Notch signaling in PCs but inhibits Notch signaling in ICs. Tfcp2l1 inactivation deletes ICs, whereas Jag1 inactivation results in the forfeiture of discrete IC and PC identities. Thus, Tfcp2l1 is a critical regulator of IC-PC patterning, acting cell-autonomously in ICs, and non-cell-autonomously in PCs. As a result, Tfcp2l1 regulates the diversification of cell types which is the central characteristic of 'salt and pepper' epithelia and distinguishes the collecting duct from all other nephron segments.

DOI:http://dx.doi.org/10.7554/eLife.24265.001

Drosophila Malpighian Tubules: A Model for Understanding Kidney Development, Function, and Disease

The Malpighian tubules of insects are structurally simple but functionally important organs, and their integrity is important for the normal excretory process. They are functional analogs of human kidneys which are important physiological organs as they maintain water and electrolyte balance in the blood and simultaneously help the body to get rid of waste and toxic products after various metabolic activities. In addition, it receives early indications of insults to the body such as immune challenge and other toxic components and is essential for sustaining life. According to National Vital Statistics Reports 2016, renal dysfunction has been ranked as the ninth most abundant cause of death in the USA. This chapter provides detailed descriptions of Drosophila Malpighian tubule development, physiology, immune function and also presents evidences that Malpighian tubules can be used as a model organ system to address the fundamental questions in developmental and functional disorders of the kidney.

Hox control of Drosophila larval anatomy; The Alary and Thoracic Alary-Related Muscles

The body plan of arthropods and vertebrates involves the formation of repetitive segments, which subsequently diversify to give rise to different body parts along the antero-posterior/rostro-caudal body axis. Anatomical variations between body segments are crucial for organ function and organismal fitness. Pioneering work in Drosophila has established that Hox transcription factors play key roles both in endowing initially identical segments with distinct identities and organogenesis. The focus of this review is on Alary Muscles (AMs) and the newly discovered Thoracic Alary-Related Muscles (TARMs). AMs and TARMs are thin muscles which together connect the circulatory system and different midgut regions to the exoskeleton, while intertwining with the respiratory tubular network. They were hypothesized to represent a new type of muscles with spring-like properties, maintaining internal organs in proper anatomical positions during larval locomotion. Both the morphology of TARMs relative to AMs, and morphogenesis of connected tissues is under Hox control, emphasizing the key role of Hox proteins in coordinating the anatomical development of the larva.

Epidermal growth factor signalling controls myosin II planar polarity to orchestrate convergent extension movements during Drosophila tubulogenesis

A study in fruit flies shows that during the elongation of embryonic renal tubules, graded signalling provides axial information for polarized myosin pulses that shorten cells circumferentially, driving intercalation of the cells and elongation of the tubule.

Most epithelial tubes arise as small buds and elongate by regulated morphogenetic processes including oriented cell division, cell rearrangements, and changes in cell shape. Through live analysis of Drosophila renal tubule morphogenesis we show that tissue elongation results from polarised cell intercalations around the tubule circumference, producing convergent-extension tissue movements. Using genetic techniques, we demonstrate that the vector of cell movement is regulated by localised epidermal growth factor (EGF) signalling from the distally placed tip cell lineage, which sets up a distal-to-proximal gradient of pathway activation to planar polarise cells, without the involvement for PCP gene activity. Time-lapse imaging at subcellular resolution shows that the acquisition of planar polarity leads to asymmetric pulsatile Myosin II accumulation in the basal, proximal cortex of tubule cells, resulting in repeated, transient shortening of their circumferential length. This repeated bias in the polarity of cell contraction allows cells to move relative to each other, leading to a reduction in cell number around the lumen and an increase in tubule length. Physiological analysis demonstrates that animals whose tubules fail to elongate exhibit abnormal excretory function, defective osmoregulation, and lethality.

Many of the tissues in our bodies are built up around complex arrays of elongated cellular tubes, which permit the entry, exit, and transport of essential molecules such as oxygen, glucose, and water. These tubes often arise as short buds, which elongate dramatically as the organ grows. We sought to understand the mechanisms that govern such transformations of shape using the fly renal tubule as a model. We find that elongation of this tissue is predominantly driven by cell rearrangement. Cells move around the circumference of the tubule, intercalating with each other so that the cell number around the lumen reduces, while increasing along the length of the tube. Our next question was how cells sense the direction in which they should move. We show that cells orient their position in the tissue by reading a signal sent out by a specific pair of cells at the tip of each tube. Cells use this directional information to make polarised movements through the asymmetric activity of the cell's contractile machinery. We find that the activity of myosin—the motor protein that regulates contraction—is pulsatile and polarised within the cell. This activity shortens the cells' circumferential lengths, so that cells move past each other around the tube circumference, thereby intercalating and producing tube elongation. We go on to show that excretory physiology is severely impaired when elongation fails, underlining the importance of sculpting organs with appropriate dimensions.

Malpighian tubule development in the red flour beetle (Tribolium castaneum)

Malpighian tubules (MpTs) are the major organ for excretion and osmoregulation in most insects. MpT development is characterised for Drosophila melanogaster, but not other species. We therefore do not know the extent to which the MpT developmental programme is conserved across insects. To redress this we provide a comprehensive description of MpT development in the beetle Tribolium castaneum (Coleoptera), a species separated from Drosophila by >315 million years. We identify similarities with Drosophila MpT development including: 1) the onset of morphological development, beginning when tubules bud from the gut and proliferate to increase organ size. 2) the tubule is shaped by convergent-extension movements and oriented cell divisions. 3) differentiated tip cells activate EGF-signalling in distal MpT cells through the ligand Spitz. 4) MpTs contain two main cell types - principal and stellate cells, differing in morphology and gene expression. We also describe development of the beetle cryptonephridial system, an adaptation for water conservation, which represents a major modification of the MpT ground plan characterised by intimate association between MpTs and rectum. This work establishes a new model to compare MpT development across insects, and provides a framework to help understand how an evolutionary novelty - the cryptonephridial system - arose during organ evolution.

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.

Tip cells act as dynamic cellular anchors in the morphogenesis of looped renal tubules in Drosophila

Tissue morphogenesis involves both the sculpting of tissue shape and the positioning of tissues relative to one another in the body. Using the renal tubules of Drosophila, we show that a specific distal tubule cell regulates both tissue architecture and position in the body cavity. Focusing on the anterior tubules, we demonstrate that tip cells make transient contacts with alary muscles at abdominal segment boundaries, moving progressively forward as convergent extension movements lengthen the tubule. Tip cell anchorage antagonizes forward-directed, TGF-β-guided tubule elongation, thereby ensuring the looped morphology characteristic of renal tubules from worms to humans. Distinctive tip cell exploratory behavior, adhesion, and basement membrane clearing underlie target recognition and dynamic interactions. Defects in these features obliterate tip cell anchorage, producing misshapen and misplaced tubules with impaired physiological function.

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Tip cells stabilize elongating renal tubules by binding to specific muscle targets

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Tip cell anchorage antagonizes migration, producing excretory tubule looping

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Tip cell exploration and adhesion dynamics underpin regulated tubule morphogenesis

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Lack of continued anchorage results in deformed tubules with impaired function

odd-skipped related 2 is required for fin chondrogenesis in zebrafish

BACKGROUND

odd-skipped related 2 (osr2) encodes a vertebrate ortholog of the Drosophila odd-skipped zinc-finger transcription factor. Osr2 in mouse is required for proper palate, eyelid, and bone development. Zebrafish knock-down experiments have also suggested a role for osr2, along with its paralog osr1, in early pectoral fin specification and pronephric development.

RESULTS

We show here that osr2 has a specific function later in development, independent of osr1, in the regulation of sox9a expression and promoting fin chondrogenesis. mRNA in situ hybridization demonstrated osr2 expression in the developing floorplate and later during organogenesis in the pronephros and gut epithelium. In the pectoral fin buds, osr2 was specifically expressed in fin mesenchyme. osr2 knock down in zebrafish embryos disrupted both three and five zinc finger alternatively spliced osr2 isoforms and eliminated wild-type osr2 mRNA. osr2 morphants exhibited normal pectoral fin bud specification but exhibited defective fin chondrogenesis, with loss of differentiated chondrocytes. Defects in chondrogenesis were paralleled by loss of sox9a as well as subsequent col2a1 expression, linking osr2 function to essential regulators of chondrogenesis.

CONCLUSIONS

The zebrafish odd-skipped related 2 gene regulates sox9a and col2a1 expression in chondrocyte development and is specifically required for zebrafish fin morphogenesis.

Homothorax plays autonomous and nonautonomous roles in proximodistal axis formation and migration of the Drosophila renal tubules

The Drosophila Malpighian tubules (MpTs) serve as a functional equivalent of the mammalian renal tubules. The MpTs are composed of two pairs of epithelial tubes that bud from the midgut-hindgut boundary during embryogenesis. The MpT primordia grow, elongate and migrate through the body cavity to assume their final position and shape. The stereotypic pattern of MpT migration is regulated by multiple intrinsic and extrinsic signals, many of which are still obscure. In this work, we implicate the TALE-class homeoprotein Homothorax (Hth) in MpT patterning. We show that in the absence of Hth the tubules fail to rearrange and migrate. Hth plays both autonomous and nonautonomous roles in this developmental process. Within the tubules Hth is required for convergent extension and for defining distal versus proximal cell identities. The difference between distal and proximal cell identities seems to be required for proper formation of the leading loop. Outside the tubules, wide-range mesodermal expression of Hth is required for directing anterior migration. The nonautonomous effects of Hth on MpT migration can be partially attributed to its effects on homeotic determination along the anterior posterior axis of the embryo and to its effects on stellate cell (SC) incorporation into the MpT.

Shaping up for action: the path to physiological maturation in the renal tubules of Drosophila

The Malpighian tubule is the main organ for excretion and osmoregulation in most insects. During a short period of embryonic development the tubules of Drosophila are shaped, undergo differentiation and become precisely positioned in the body cavity, so they become fully functional at the time of larval hatching a few hours later. In this review I explore three developmental events on the path to physiological maturation. First, I examine the molecular and cellular mechanisms that generate organ shape, focusing on the process of cell intercalation that drives tubule elongation, the roles of the cytoskeleton, the extracellular matrix and how intercalation is coordinated at the tissue level. Second, I look at the genetic networks that control the physiological differentiation of tubule cells and consider how distinctive physiological domains in the tubule are patterned. Finally, I explore how the organ is positioned within the body cavity and consider the relationship between organ position and function.

Wnt-Notch signalling: an integrated mechanism regulating transitions between cell states

The activity of Wnt and Notch signalling is central to many cell fate decisions during development and to the maintenance and differentiation of stem cell populations in homeostasis. While classical views refer to these pathways as independent signal transduction devices that co-operate in different systems, recent work has revealed intricate connections between their components. These observations suggest that rather than operating as two separate pathways, elements of Wnt and Notch signalling configure an integrated molecular device whose main function is to regulate transitions between cell states in development and homeostasis. Here, we propose a general framework for the structure and function of the interactions between these signalling systems that is focused on the notion of 'transition states', i.e. intermediates that arise during cell fate decision processes. These intermediates act as checkpoints in cell fate decision processes and are characterised by the mixed molecular identities of the states involved in these processes.

Insect response to alphavirus infection--establishment of alphavirus persistence in insect cells involves inhibition of viral polyprotein cleavage

Alphavirus persistence in the insect vector is an essential element in the vector-host transmission cycle of the virus and provides a model to study the biochemical and molecular basis for virus-vector coexistence. The prototype alphavirus Sindbis (SV) establishes persistent infections in invertebrate cell cultures which are characterized by low levels of virus production. We hypothesized that antiviral factors may be involved in decreasing the virus levels as virus persistence is established in invertebrate cells. Transcription profiles in Drosophila S2 cells at 5 days post-infection with SV identified families of gene products that code for factors that can explain previous observations seen in insect cells infected with alphaviruses. Genomic array analysis identified up-regulation of gene products involved in intracellular membrane vesicle formation, cell growth rate changes and immune-related functions in S2 cells infected with SV. Transcripts coding for factors involved in different aspects of the Notch signaling pathway had increased in expression. Increased expression of ankyrin, plap, syx13, unc-13, csp, rab1 and rab8 may aid in formation of virus containing vesicles and in intracellular transport of viral structural proteins. Possible functions of these gene products and relevant hypotheses are discussed. We confirmed the up-regulation of a wide-spectrum protease inhibitor, Thiol-ester containing Protein (TEP) II. We report inhibition of the viral polyprotein cleavage at 5 days post-infection (dpi) and after superinfection of SV-infected cells at 5 dpi. We propose that inefficient cleavage of the polyprotein may, at least in part, lead to reduced levels of virus seen as persistence is established.

The developmental, molecular, and transport biology of Malpighian tubules

Molecular biology is reaching new depths in our understanding of the development and physiology of Malpighian tubules. In Diptera, Malpighian tubules derive from ectodermal cells that evaginate from the primitive hindgut and subsequently undergo a sequence of orderly events that culminates in an active excretory organ by the time the larva takes its first meal. Thereafter, the tubules enlarge by cell growth. Just as modern experimental strategies have illuminated the development of tubules, genomic, transcriptomic, and proteomic studies have uncovered new tubule functions that serve immune defenses and the breakdown and renal clearance of toxic substances. Moreover, genes associated with specific diseases in humans are also found in flies, some of which, astonishingly, express similar pathophenotypes. However, classical experimental approaches continue to show their worth by distinguishing between -omic possibilities and physiological reality while providing further detail about the rapid regulation of the transport pathway through septate junctions and the reversible assembly of proton pumps.

Multipotent stem cells in the Malpighian tubules of adult Drosophila melanogaster

Excretion is an essential process of an organism's removal of the waste products of metabolism to maintain a constant chemical composition of the body fluids despite changes in the external environment. Excretion is performed by the kidneys in vertebrates and by Malpighian tubules (MTs) in Drosophila. The kidney serves as an excellent model organ to investigate the cellular and molecular mechanisms underlying organogenesis. Mammals and Drosophila share common principles of renal development. Tissue homeostasis, which is accomplished through self-renewal or differentiation of stem cells, is critical for the maintenance of adult tissues throughout the lifetime of an animal. Growing evidence suggests that stem cell self-renewal and differentiation is controlled by both intrinsic and extrinsic factors. Deregulation of stem cell behavior results in cancer formation, tissue degeneration, and premature aging. The mammalian kidney has a low rate of cellular turnover but has a great capacity for tissue regeneration following an ischemic injury. However, there is an ongoing controversy about the source of regenerating cells in the adult kidney that repopulate injured renal tissues. Recently, we identified multipotent stem cells in the MTs of adult Drosophila and found that these stem cells are able to proliferate and differentiate in several types of cells in MTs. Furthermore, we demonstrated that an autocrine JAK-STAT (Janus kinase-signal transducers and activators of transcription) signaling regulates stem cell self-renewal or differentiation of renal stem cells. The Drosophila MTs provide an excellent in vivo system for studying the renal stem cells at cellular and molecular levels. Understanding the molecular mechanisms governing stem cell self-renewal or differentiation in vivo is not only crucial to using stem cells for future regenerative medicine and gene therapy, but it also will increase our understanding of the mechanisms underlying cancer formation, aging and degenerative diseases. Identifying and understanding the cellular processes underlying the development and repair of the mammalian kidney may enable more effective, targeted therapies for acute and chronic kidney diseases in humans.

Timing cell-fate determination during asymmetric cell divisions

From invertebrates to mammals, cell-cycle progression during an asymmetric cell division is accompanied by precisely timed redistribution of cell-fate determinants so that they segregate asymmetrically to enable the two daughter cells to choose different fates. Interestingly, studies on how cell fates are specified in such divisions reveal that the same fate determinants can be reiteratively used to specify a variety of cell types through multiple rounds of cell divisions or to exert seemingly contradictory effects on cell proliferation and differentiation. Here I summarize the molecular mechanisms governing asymmetric cell division and review recent findings pointing to a novel mechanism for coupling intracellular signaling and cell-cycle progression. This mechanism uses changes in the morphology, subcellular distribution, and molecular composition of cellular organelles like the Golgi apparatus and centrosomes, which not only accompany the progression of cell cycle to activate but also temporally constrain the activity of fate determinants during asymmetric cell divisions.

Wnt/Notch signalling and information processing during development

The Wnt and Notch signalling pathways represent two major channels of communication used by animal cells to control their identities and behaviour during development. A number of reports indicate that their activities are closely intertwined during embryonic development. Here, we review the evidence for this relationship and suggest that Wnt and Notch ('Wntch') signalling act as components of an integrated device that, rather than defining the fate of a cell, determines the probability that a cell will adopt that fate.

Regulation of vascular morphogenesis by Notch signaling

The Notch pathway is a versatile regulator of cell fate specification, growth, differentiation, and patterning processes in metazoan organisms. In the vertebrate cardiovascular system, multiple Notch family receptors and several of their Jagged and Delta-like ligands are expressed during critical stages of embryonic and postnatal development. Functional studies in mice, fish, tumor models, and cell culture systems have shown that the angiogenic growth of the blood vessel network, the proliferation of endothelial cells, and the differentiation of arteries and veins are controlled by Notch signaling. Moreover, Notch pathway components play important roles in human pathological conditions involving the vasculature, namely CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy) and Alagille syndrome. Recent findings highlight the Notch ligand Delta-like 4 as a key regulator of tumor angiogenesis and suggest that this protein might be a promising target for cancer therapy.

Expression of the adaptor protein m-Numb in mouse male germ cells

Numb is an adaptor protein that is asymmetrically inherited at mitosis and controls the fate of sibling cells in different species. The role of m-Numb (mammalian Numb) as an important cell fate-determining factor has extensively been described mostly in neural tissues, particularly in progenitor cells, in the mouse. Biochemical and genetic analyses have shown that Numb acts as an inhibitor of the Notch signaling pathway, an evolutionarily conserved pathway involved in the control of cell proliferation, differentiation, and apoptosis. In the present study, we sought to determine m-Numb distribution in germ cells in the postnatal mouse testis. We show that all four m-Numb isoforms are widely expressed during postnatal testis development. By reverse transcriptase-PCR and western blot analyses, we further identify p71 as the predominantly expressed isoform in germ cells. Moreover, we demonstrate through co-immunoprecipitation studies that m-Numb physically associates with Ap2a1, a component of the endocytotic clathrin-coated vesicles. Finally, we employed confocal immunofluorescence microscopy of whole mount seminiferous tubules and isolated germ cells to gain more insight into the subcellular localization of m-Numb. These morphological analyses confirmed m-Numb and Ap2a1 co-localization. However, we did not observe asymmetric localization of m-Numb neither in mitotic spermatogonial stem cells nor in more differentiated spermatogonial cells, suggesting that spermatogonial stem cell fate in the mouse does not rely on asymmetric partitioning of m-Numb.

Odd-skipped genes encode repressors that control kidney development

Odd-skipped family of proteins (Odd in Drosophila and Osr in vertebrates) are evolutionarily conserved zinc finger transcription factors. Two Osr genes are present in mammalian genomes, and it was recently reported that Osr1, but not Osr2, is required for murine kidney development. Here, we show that in Xenopus and zebrafish both Osr1 and Osr2 are necessary and sufficient for the development of the pronephros. Osr genes are expressed in early prospective pronephric territories, and morphants for either of the two genes show severely impaired kidney development. Conversely, overexpression of Osr genes promotes formation of ectopic kidney tissue. Molecularly, Osr proteins function as transcriptional repressors during kidney formation. We also show that Drosophila Odd induces kidney tissue in Xenopus. This might be accomplished through recruitment of Groucho-like co-repressors. Odd genes may also be required for proper development of the Malpighian tubules, the Drosophila renal organs. Our results highlight the evolutionary conserved involvement of Odd-skipped transcription factors in the development of kidneys.

The Enigma of the Numb-Notch Relationship during Mammalian Embryogenesis

Asymmetric cell division is an attractive means to diversify cell fates during development and for stem cells to balance self-renewal and differentiation. In Drosophila, two signaling pathways, one mediated by Numb and the other by Notch, play essential but antagonistic roles in enabling the two daughters to adopt different fates after a wide variety of asymmetric cell divisions. However, recent studies show that mutating mammalian Numb homologues, m-Numb and Numblike (Numbl or Nbl), and eliminating Notch signaling in the developing nervous system both lead to premature depletion of neural stem/progenitor cells in mice. These findings raise an interesting question as to whether and how the antagonistic roles of Numb and Notch signaling are conserved in vertebrates. Here we provide evidence that loss of m-Numb and Numbl outside the embryonic nervous system also causes phenotypes similar to those exhibited by mice with defective Notch signaling. We further show that very little Numb protein is necessary for embryogenesis and that the presence of different m-Numb isoforms is unlikely to provide a molecular basis for differential regulation of Notch signaling in mammals, as these isoforms are functionally indistinguishable in cell fate specification. We discuss possible mechanisms by which the antagonistic roles of Numb and Notch are evolutionarily conserved to meet the needs of stem cell maintenance during mammalian neurogenesis.

Filtering transcriptional noise during development: concepts and mechanisms

The assignation of cell fates during eukaryotic development relies on the coordinated and stable expression of cohorts of genes within cell populations. The precise and reproducible nature of this process is remarkable given that, at the single-cell level, the transcription of individual genes is associated with noise - random molecular fluctuations that create variability in the levels of gene expression within a cell population. Here we consider the implications of transcriptional noise for development and suggest the existence of molecular devices that are dedicated to filtering noise. On the basis of existing evidence, we propose that one such mechanism might depend on the Wnt signalling pathway.

In Drosophila melanogaster, the rolling pebbles isoform 6 (Rols6) is essential for proper Malpighian tubule morphology

During myoblast fusion, cell-cell recognition along with cell migration and adhesion are essential biological processes. The factors involved in these processes include members of the immunoglobulin superfamily like Sticks and stones (Sns), Dumbfounded (Duf) and Hibris (Hbs), SH3 domain-containing adaptor molecules like Myoblast city (Mbc) and multidomain proteins like Rolling pebbles (Rols). For rolling pebbles, two differentially expressed transcripts have been defined (rols7 and rols6). However, to date, only a muscle fusion phenotype has been described and assigned to the lack of the mesoderm-specific expressed rols7 transcript. Here, we show that a loss of the second rolling pebbles transcript, rols6, which is expressed from the early bud to later embryonic stages during Malpighian tubule (MpT) development, leads to an abnormal MpT morphology that is not due to defects in cell determination or proliferation but to aberrant morphogenesis. In addition, when Myoblast city or Rac are knocked out, a similar phenotype is observed. Myoblast city and Rac are essentially involved in the development of the somatic muscles and were proposed to be interaction partners of Rols7. Because of the predicted structural similarities of the Rols7 and Rols6 proteins, we argue that genetic interaction of rols6, mbc and rac might lead to proper MpT morphology. We also propose that these interactions result in stable cell connections due to rearrangement of the cytoskeleton.

Expression of Hairy/Enhancer of Split genes, Hes1 and Hes5, during murine nephron morphogenesis

Hairy/Enhancer of Split (Hes) genes encode transcriptional repressors that function as downstream targets of activated Notch receptors in cell fate decisions during tissue development. During nephrogenesis, multiple Notch pathway genes are co-expressed in multi-potent epithelial progenitors (i.e. pre-tubular aggregates), but demonstrate distinct expression patterns in early nephrons (i.e. S-shaped bodies), suggesting that Notch signaling functions in patterning epithelial cell fate during nephron morphogenesis. To define the spatial activation of the Notch pathway in developing nephrons, we analyzed the expression of Hes1 and Hes5 by mRNA in situ hybridization in cryosections of developing kidneys, and compared their spatiotemporal expression with the expression of other Notch pathway genes in nephron progenitors. Hes1, and to a lesser extent Hes5, were expressed in pre-tubular aggregates and comma-shaped bodies of embryonic day (E) 13.5 and newborn kidneys. In S-shaped bodies, Hes1 expression was detected in the middle part which gives rise to the proximal tubule, but also extended into the lower and upper parts which give rise to the glomerulus and distal tubule, respectively, and was similar to the proximal-distal expression patterns for Notch1 and Jagged1 in these nephrogenic structures. In contrast, strong Hes5 expression was restricted to the middle segment of S-shaped bodies, and resembled Delta-like 1 expression. These data show that Hes1 and Hes5 expression are independently regulated along the proximal-distal axis of the developing nephron. Consequently, the differential, spatial regulation of Hes1 and Hes5 gene expression by the Notch signaling pathway in developing nephrons may be a mechanism for patterning cell fate decisions during nephron morphogenesis.

Function-informed transcriptome analysis of Drosophila renal tubule

Analysis of the transcriptome of the Drosophila melanogaster Malpighian (renal) tubule gives a radically new view of the function of the tubule, emphasising solute transport rather than fluid secretion.

Background

Comprehensive, tissue-specific, microarray analysis is a potent tool for the identification of tightly defined expression patterns that might be missed in whole-organism scans. We applied such an analysis to Drosophila melanogaster Malpighian (renal) tubule, a defined differentiated tissue.

Results

The transcriptome of the D. melanogaster Malpighian tubule is highly reproducible and significantly different from that obtained from whole-organism arrays. More than 200 genes are more than 10-fold enriched and over 1,000 are significantly enriched. Of the top 200 genes, only 18 have previously been named, and only 45% have even estimates of function. In addition, 30 transcription factors, not previously implicated in tubule development, are shown to be enriched in adult tubule, and their expression patterns respect precisely the domains and cell types previously identified by enhancer trapping. Of Drosophila genes with close human disease homologs, 50 are enriched threefold or more, and eight enriched 10-fold or more, in tubule. Intriguingly, several of these diseases have human renal phenotypes, implying close conservation of renal function across 400 million years of divergent evolution.

Conclusions

From those genes that are identifiable, a radically new view of the function of the tubule, emphasizing solute transport rather than fluid secretion, can be obtained. The results illustrate the phenotype gap: historically, the effort expended on a model organism has tended to concentrate on a relatively small set of processes, rather than on the spread of genes in the genome.

A hidden program in Drosophila peripheral neurogenesis revealed: fundamental principles underlying sensory organ diversity

How is cell fate diversity reliably achieved during development? Insect sensory organs have been a favorable model system for investigating this question for over 100 years. They are constructed using defined cell lineages that generate a maximum of cell diversity with a minimum number of cell divisions, and display tremendous variety in their morphologies, constituent cell types, and functions. An unexpected realization of the past 5 years is that very diverse sensory organs in Drosophila are produced by astonishingly similar cell lineages, and that their diversity can be largely attributed to only a small repertoire of developmental processes. These include changes in terminal cell differentiation, cell death, cell proliferation, cell recruitment, cell-cell interactions, and asymmetric segregation of cell fate determinants during mitosis. We propose that most Drosophila sensory organs are built from an archetypal lineage, and we speculate about how this stereotyped pattern of cell divisions may have been built during evolution.

Murine numb regulates granule cell maturation in the cerebellum

Notch is a key regulator of vertebrate neurogenesis and the cytoplasmic adaptor protein Numb is a modulator of the Notch signaling pathway. To address the role of murine Numb in development of the central nervous system, we used a conditional gene ablation approach. We show that Numb is involved in the maturation of cerebellar granule cells. Although the specification of neural cell fates in the cerebellum is not affected in the absence of Numb, the transition from a mitotic progenitor to a mature granule cell is aberrant and migration of postmitotic granule cells to the internal granule cell layer is delayed. In some animals, this results in a complete agenesis of granule cells and a strong ataxia. We confirmed these findings in vitro and found that Numb-deficient cerebellar progenitor cells show a marked delay in granule cell maturation. Our results suggest that Numb plays a role in the transition of a mitotic progenitor to a fully differentiated granule cell in the cerebellum. In addition, the maturation of Purkinje cells is also delayed in Numb-deficient mice.

The signals that drive kidney development: a view from the fly eye

PURPOSE OF REVIEW

Development of the mammalian kidney is a complex process involving numerous signals and signaling pathways. Other complex tissues have benefited enormously from studies in lower, simpler organisms. The present review provides an update on what we have learned from the fruitfly Drosophila melanogaster, and argues that Drosophila is an important but under-utilized organism for study of renal development.

RECENT FINDINGS

The Malpighian tubules provide renal function to the fly. These require a number of signaling pathways for their development that are also seen in vertebrate kidney development, including the Notch, Ras, and Wnt signaling pathways, as well as nuclear factors such as Krüppel and Cut/Cux-1. Many of these factors are shared between early Malpighian tubule development and ureteric bud formation. The Ret signaling receptor, which is central to mammalian renal development, is poorly understood in flies, although its expression pattern is intriguing. Surprisingly, other signaling factors such as Neph-1, Pax2, and Wilms' tumor suppressor-1 appear to work within later fly retinal development, providing a surprising link between these two disparate tissues.

SUMMARY

Drosophila offers a powerful palate of tools for dissecting developmental processes. Importantly, these tools can often be examined at the level of single cells, permitting us to address issues of differentiation with high resolution. If we are to take full advantage of Drosophila, however, then we must target specific issues and gain a better understanding of the details of Malpighian tubule development.

Creating asymmetric cell divisions by skewing endocytosis

How can one cell divide to form two cells, which are genetically identical to the mother cell, with different cell fates? That is the question discussed by Shen and Temple. Specifically, they address how Notch signaling is attenuated in one daughter cell through the asymmetric inheritance of the protein Numb, which may be responsible for mediating the endocytosis of the receptor Notch in the Numb-enriched cell.

Asymmetric segregation of Numb: a mechanism for neural specification from Drosophila to mammals

It is a major challenge to understand how the neuroepithelial cells of the developing CNS choose between alternative cell fates to generate cell diversity. In invertebrates such as Drosophila melanogaster and Caenorhabditis elegans, asymmetric segregation of cell-fate determining proteins or mRNAs to the two daughter cells during precursor cell division plays a crucial part in cell diversification. There is increasing evidence that this mechanism also operates in vertebrate neural development and that Numb proteins, which function as cell-fate determinants during Drosophila development, may also function in this way in vertebrates. Recent studies on mouse cortical progenitor cells have provided the strongest evidence yet that this is the case. Here, we review these and other findings that suggest an important role for the asymmetric segregation of Numb proteins in vertebrate neural development.

ITF-2, a downstream target of the Wnt/TCF pathway, is activated in human cancers with beta-catenin defects and promotes neoplastic transformation

In many cancers, inactivation of the adenomatous polyposis coli (APC) or Axin tumor suppressor proteins or activating mutations in beta-catenin lead to elevated beta-catenin levels, enhanced binding of beta-catenin to T cell factor (TCF) proteins, and increased expression of TCF-regulated genes. We found that the gene for the basic helix-loop-helix transcription factor ITF-2 (immunoglobulin transcription factor-2) was activated in rat E1A-immortalized RK3E cells following neoplastic transformation by beta-catenin or ligand-induced activation of a beta-catenin-estrogen receptor fusion protein. Human cancers with beta-catenin regulatory defects had elevated ITF-2 expression, and ITF-2 was repressed by restoring wild-type APC function or inhibiting TCF activity. Of note, ITF-2 promoted neoplastic transformation of RK3E cells. We propose that ITF-2 is a TCF-regulated gene, which functions in concert with other TCF target genes to promote growth and/or survival of cancer cells with defects in beta-catenin regulation.

A genetic hierarchy establishes mitogenic signalling and mitotic competence in the renal tubules of Drosophila

Cell proliferation in the developing renal tubules of Drosophila is strikingly patterned, occurring in two phases to generate a consistent number of tubule cells. The later phase of cell division is promoted by EGF receptor signalling from a specialised subset of tubule cells, the tip cells, which express the protease Rhomboid and are thus able to secrete the EGF ligand, Spitz. We show that the response to EGF signalling, and in consequence cell division, is patterned by the specification of a second cell type in the tubules. These cells are primed to respond to EGF signalling by the transcription of two pathway effectors, PointedP2, which is phosphorylated on pathway activation, and Seven up. While expression of pointedP2 is induced by Wingless signalling, seven up is initiated in a subset of the PointedP2 cells through the activity of the proneural genes. We demonstrate that both signalling and responsive cells are set aside in each tubule primordium from a proneural gene-expressing cluster of cells, in a two-step process. First, a proneural cluster develops within the domain of Wingless-activated, pointedP2-expressing cells to initiate the co-expression of seven up. Second, lateral inhibition, mediated by the neurogenic genes, acts within this cluster of cells to segregate the tip cell precursor, in which proneural gene expression strengthens to initiate rhomboid expression. As a consequence, when the precursor cell divides, both daughters secrete Spitz and become signalling cells. Establishing domains of cells competent to transduce the EGF signal and divide ensures a rapid and reliable response to mitogenic signalling in the tubules and also imposes a limit on the extent of cell division, thus preventing tubule hyperplasia.

Novel Notch alleles reveal a Deltex-dependent pathway repressing neural fate

BACKGROUND

The Notch receptor triggers a wide range of cell fate choices in higher organisms. In Drosophila, segregation of neural from epidermal lineages results from competition among equivalent cells. These cells express achaete/scute genes, which confer neural potential. During lateral inhibition, a single neural precursor is selected, and neighboring cells are forced to adopt an epidermal fate. Lateral inhibition relies on proteolytic cleavage of Notch induced by the ligand Delta and translocation of the Notch intracellular domain (NICD) to the nuclei of inhibited cells. The activated NICD, interacting with Suppressor of Hairless [Su(H)], stimulates genes of the E(spl) complex, which in turn repress the proneural genes achaete/scute.

RESULTS

Here, we describe new alleles of Notch that specifically display loss of microchaetae sensory precursors. This phenotype arises from a repression of neural fate, by a Notch signaling distinct from that involved in lateral inhibition. We show that the loss of sensory organs associated with this phenotype results from a constitutive activation of a Deltex-dependent Notch-signaling event. These novel Notch alleles encode truncated receptors lacking the carboxy terminus of the NICD, which is the binding site for the repressor Dishevelled (Dsh). Dsh is known to be involved in crosstalk between Wingless and Notch pathways.

CONCLUSIONS

Our results reveal an antineural activity of Notch distinct from lateral inhibition mediated by Su(H). This activity, mediated by Deltex (Dx), represses neural fate and is antagonized by elements of the Wingless (Wg)-signaling cascade to allow alternative cell fate choices.

Asymmetric segregation of Numb in retinal development and the influence of the pigmented epithelium

Asymmetric segregation of cell-fate determinants during cytokinesis plays an important part in controlling cell-fate choice in invertebrates. During Drosophila neurogenesis, for example, asymmetric segregation of the Numb protein, which inhibits Notch signaling, is necessary for the two daughter cells of a division to have different fates. In vertebrates, the role of asymmetric segregation of cell-fate determinants is uncertain, and the way the process might be regulated is unknown. We have studied the orientation of cell divisions and the distribution of Numb in the developing rat retina. We show that, whereas most retinal neuroepithelial cells divide with their mitotic spindles oriented parallel to the plane of the neuroepithelium, a substantial minority divides with their spindles oriented perpendicularly. The proportion of these vertically dividing cells changes during development, peaking around the day of birth. Numb appears to be inherited only by the apical daughter cell when a neuroepithelial cell divides vertically. Similarly, in dissociated cell cultures, some retinal neuroepithelial cells divide asymmetrically and distribute Numb to only one of the two daughter cells, suggesting that the dissociated cells can retain their polarity in vitro. Using retinal explant cultures, we find that the retinal pigment epithelium apparently promotes vertical divisions in the neural retina. To our knowledge, this is the first evidence that asymmetric segregation of cell-fate determinants may contribute to cell diversification in the mammalian retina and that an epithelium controls this process by influencing the plane of division in the adjacent neural retina.

ribbon encodes a novel BTB/POZ protein required for directed cell migration in Drosophila melanogaster

During development, directed cell migration is crucial for achieving proper shape and function of organs. One well-studied example is the embryonic development of the larval tracheal system of Drosophila, in which at least four signaling pathways coordinate cell migration to form an elaborate branched network essential for oxygen delivery throughout the larva. FGF signaling is required for guided migration of all tracheal branches, whereas the DPP, EGF receptor, and Wingless/WNT signaling pathways each mediate the formation of specific subsets of branches. Here, we characterize ribbon, which encodes a BTB/POZ-containing protein required for specific tracheal branch migration. In ribbon mutant tracheae, the dorsal trunk fails to form, and ventral branches are stunted; however, directed migrations of the dorsal and visceral branches are largely unaffected. The dorsal trunk also fails to form when FGF or Wingless/WNT signaling is lost, and we show that ribbon functions downstream of, or parallel to, these pathways to promote anterior-posterior migration. Directed cell migration of the salivary gland and dorsal epidermis are also affected in ribbon mutants, suggesting that conserved mechanisms may be employed to orient cell migrations in multiple tissues during development.

Multiple roles of mouse Numb in tuning developmental cell fates

BACKGROUND

Notch signaling regulates multiple differentiation processes and cell fate decisions during both invertebrate and vertebrate development. Numb encodes an intracellular protein that was shown in Drosophila to antagonize Notch signaling at binary cell fate decisions of certain cell lineages. Although overexpression experiments suggested that Numb might also antagonize some Notch activity in vertebrates, the developmental processes in which Numb is involved remained elusive.

RESULTS

We generated mice with a homozygous inactivation of Numb. These mice died before embryonic day E11.5, probably because of defects in angiogenic remodeling and placental dysfunction. Mutant embryos had an open anterior neural tube and impaired neuronal differentiation within the developing cranial central nervous system (CNS). In the developing spinal cord, the number of differentiated motoneurons was reduced. Within the peripheral nervous system (PNS), ganglia of cranial sensory neurons were formed. Trunk neural crest cells migrated and differentiated into sympathetic neurons. In contrast, a selective differentiation anomaly was observed in dorsal root ganglia, where neural crest--derived progenitor cells had migrated normally to form ganglionic structures, but failed to differentiate into sensory neurons.

CONCLUSIONS

Mouse Numb is involved in multiple developmental processes and required for cell fate tuning in a variety of lineages. In the nervous system, Numb is required for the generation of a large subset of neuronal lineages. The restricted requirement of Numb during neural development in the mouse suggests that in some neuronal lineages, Notch signaling may be regulated independently of Numb.

Numb is an endocytic protein

Numb is a protein that in Drosophila determines cell fate as a result of its asymmetric partitioning at mitosis. The function of Numb has been linked to its ability to bind and to biologically antagonize Notch, a membrane receptor that also specifies cell fate. The biochemical mechanisms underlying the action of Numb, however, are still largely unknown. The wide pattern of expression of Numb suggests a general function in cellular homeostasis that could be additional to, or part of, its action in fate determination. Such a function could be endocytosis, as suggested by the interaction of Numb with Eps15, a component of the endocytic machinery. Here, we demonstrate that Numb is an endocytic protein. We found that Numb localizes to endocytic organelles and is cotrafficked with internalizing receptors. Moreover, it associates with the appendage domain of alpha adaptin, a subunit of AP2, a major component of clathrin-coated pits. Finally, fragments of Numb act as dominant negatives on both constitutive and ligand-regulated receptor-mediated internalization, suggesting a general role for Numb in the endocytic process.

Notch regulates cell fate in the developing pronephros

The mechanisms that regulate cell fate within the pronephros are poorly understood but are important for the subsequent development of the urogenital system and show many similarities to nephrogenesis in the definitive kidney. Dynamic expression of Notch-1, Serrate-1, and Delta-1 in the developing Xenopus pronephros suggests a role for this pathway in cell fate segregation. Misactivation of Notch signaling using conditionally active forms of either Notch-1 or RBP-J/Su(H) proteins prevented normal duct formation and the proper expression of genetic markers of duct cell differentiation. Inhibition of endogenous Notch signaling elicited the opposite effect. Taken together with the mRNA expression patterns, these data suggest that endogenous Notch signaling functions to inhibit duct differentiation in the dorsoanterior region of the anlage where cells are normally fated to form tubules. In addition, elevated Notch signaling in the pronephric anlage both perturbed the characteristic pattern of the differentiated tubule network and increased the expression of early markers of pronephric precursor cells, Pax-2 and Wilms' tumor suppressor gene (Wt-1). We propose that Notch signaling plays a previously unrecognized role in the early selection of duct and tubule cell fates as well as functioning subsequently to control tubule cell patterning and development.

The informational content of gradients of Wnt proteins

This perspective tackles the issues facing developmental biologists and cell biologists regarding how the molecular mechanisms for specifying cell fate are defined. This perspective focuses on members of the Wnt family. The author proposes that Wnt proteins may act as stabilizing signals for earlier inductive events in certain systems, for example, in Caenorhabditis elegans during the migration of two neurons and in Drosophila melanogaster during the patterning of the wing.

Coordinating cell fate and morphogenesis in Drosophila renal tubules

Using the renal tubules of Drosophila as an example, we explore how cell specification leads to the morphogenetic movements that underlie the generation of tissue architecture. Taking two stages of development, we show first that the tubule cells are allocated by signalling between the endodermal and ectodermal compartments of the posterior gut. Activation of the Wnt pathway patterns the ectodermal anlage, resulting in the expression of tubule genes in a subset of cells and their eversion from the hindgut to form the tubule primordia. We argue that early gene expression directs these morphogenetic movements but not the complete programme of tubule differentiation. In the second example we show that the allocation of the mitogenic tip cell lineage in each tubule is required not only for the normal pattern of cell division but also for the stereotyped three-dimensional arrangement of the mature tubules. Analysis of mutants in which the tip cell lineage is misspecified reveals that both daughters of the tip cell progenitor are required for the tubules to navigate through the body cavity, so that the distal tips locate in their characteristic positions. We show that the regulator of Rac, Myoblast city is essential for this second morphogenetic process.

Mouse numb is an essential gene involved in cortical neurogenesis

During neurogenesis of the mammalian neocortex, neural progenitor cells divide to generate daughter cells that either become neurons or remain as progenitor cells. The mouse numb (m-numb) gene encodes a membrane-associated protein that is asymmetrically localized to the apical cell membrane of dividing cortical progenitor cells and may be segregated to only the apical daughter cell that has been suggested to remain as a progenitor cell. To examine m-numb function during neural development, we generated a loss-of-function mutant allele of m-numb. Mice homozygous for this mutation exhibit severe defects in cranial neural tube closure and precocious neuron production in the forebrain and die around embryonic day 11.5 (E11. 5). These findings suggest that m-numb is an essential gene that plays a role in promoting progenitor cell fate during cortical neurogenesis.