Real-time imaging of morphogenetic movements in Drosophila using Gal4-UAS-driven expression of GFP fused to the actin-binding domain of moesin

An acytokinetic cell division creates PIP2-enriched membrane asymmetries leading to slit diaphragm assembly in Drosophila nephrocytes

Vertebrate podocytes and Drosophila nephrocytes display slit diaphragms, specialised cell junctions that are essential for the execution of the basic excretory function of ultrafiltration. To elucidate the mechanisms of slit diaphragm assembly we have studied their formation in Drosophila embryonic garland nephrocytes. These cells of mesenchymal origin lack overt apical-basal polarity. We find that their initial membrane symmetry is broken by an acytokinetic cell division that generates PIP2-enriched domains at their equator. The PIP2-enriched equatorial cortex becomes a favourable domain for hosting slit diaphragm proteins and the assembly of the first slit diaphragms. Indeed, when this division is either prevented or forced to complete cytokinesis, the formation of diaphragms is delayed to larval stages. Furthermore, although apical polarity determinants also accumulate at the equatorial cortex, they do not appear to participate in the recruitment of slit diaphragm proteins. The mechanisms we describe allow the acquisition of functional nephrocytes in embryos, which may confer on them a biological advantage similar to the formation of the first vertebrate kidney, the pronephros.

Drosophila Dendritic Arborisation Neurons: Fantastic Actin Dynamics and Where to Find Them

Neuronal dendrites receive, integrate, and process numerous inputs and therefore serve as the neuron's "antennae". Dendrites display extreme morphological diversity across different neuronal classes to match the neuron's specific functional requirements. Understanding how this structural diversity is specified is therefore important for shedding light on information processing in the healthy and diseased nervous system. Popular models for in vivo studies of dendrite differentiation are the four classes of dendritic arborization (c1da-c4da) neurons of Drosophila larvae with their class-specific dendritic morphologies. Using da neurons, a combination of live-cell imaging and computational approaches have delivered information on the distinct phases and the time course of dendrite development from embryonic stages to the fully developed dendritic tree. With these data, we can start approaching the basic logic behind differential dendrite development. A major role in the definition of neuron-type specific morphologies is played by dynamic actin-rich processes and the regulation of their properties. This review presents the differences in the growth programs leading to morphologically different dendritic trees, with a focus on the key role of actin modulatory proteins. In addition, we summarize requirements and technological progress towards the visualization and manipulation of such actin regulators in vivo.

Scaling of internal organs during Drosophila embryonic development

Many species show a diverse range of sizes; for example, domestic dogs have large variation in body mass. Yet, the internal structure of the organism remains similar, i.e., the system scales to organism size. Drosophila melanogaster has been a powerful model system for exploring scaling mechanisms. In the early embryo, gene expression boundaries scale very precisely to embryo length. Later in development, the adult wings grow with remarkable symmetry and scale well with animal size. Yet, our knowledge of whether internal organs initially scale to embryo size remains largely unknown. Here, we utilize artificially small Drosophila embryos to explore how three critical internal organs-the heart, hindgut, and ventral nerve cord (VNC)-adapt to changes in embryo morphology. We find that the heart scales precisely with embryo length. Intriguingly, reduction in cardiac cell length, rather than number, appears to be important in controlling heart length. The hindgut, which is the first chiral organ to form, displays scaling with embryo size under large-scale changes in the artificially smaller embryos but shows few hallmarks of scaling within wild-type size variation. Finally, the VNC only displays weak scaling behavior; even large changes in embryo geometry result in only small shifts in VNC length. This suggests that the VNC may have an intrinsic minimal length that is largely independent of embryo length. Overall, our work shows that internal organs can adapt to embryo size changes in Drosophila, but the extent to which they scale varies significantly between organs.

Distinct Relations of Microtubules and Actin Filaments with Dendritic Architecture

Microtubules (MTs) and F-actin (F-act) have long been recognized as key regulators of dendritic morphology. Nevertheless, precisely ascertaining their distinct influences on dendritic trees have been hampered until now by the lack of direct, arbor-wide cytoskeletal quantification. We pair live confocal imaging of fluorescently labeled dendritic arborization (da) neurons in Drosophila larvae with complete multi-signal neural tracing to separately measure MTs and F-act. We demonstrate that dendritic arbor length is highly interrelated with local MT quantity, whereas local F-act enrichment is associated with dendritic branching. Computational simulation of arbor structure solely constrained by experimentally observed subcellular distributions of these cytoskeletal components generated synthetic morphological and molecular patterns statistically equivalent to those of real da neurons, corroborating the efficacy of local MT and F-act in describing dendritic architecture. The analysis and modeling outcomes hold true for the simplest (class I), most complex (class IV), and genetically altered (Formin3 overexpression) da neuron types.

The apical protein Apnoia interacts with Crumbs to regulate tracheal growth and inflation

Most organs of multicellular organisms are built from epithelial tubes. To exert their functions, tubes rely on apico-basal polarity, on junctions, which form a barrier to separate the inside from the outside, and on a proper lumen, required for gas or liquid transport. Here we identify apnoia (apn), a novel Drosophila gene required for tracheal tube elongation and lumen stability at larval stages. Larvae lacking Apn show abnormal tracheal inflation and twisted airway tubes, but no obvious defects in early steps of tracheal maturation. apn encodes a transmembrane protein, primarily expressed in the tracheae, which exerts its function by controlling the localization of Crumbs (Crb), an evolutionarily conserved apical determinant. Apn physically interacts with Crb to control its localization and maintenance at the apical membrane of developing airways. In apn mutant tracheal cells, Crb fails to localize apically and is trapped in retromer-positive vesicles. Consistent with the role of Crb in apical membrane growth, RNAi-mediated knockdown of Crb results in decreased apical surface growth of tracheal cells and impaired axial elongation of the dorsal trunk. We conclude that Apn is a novel regulator of tracheal tube expansion in larval tracheae, the function of which is mediated by Crb.

RNA Interference Screening for Genes Regulating Drosophila Muscle Morphogenesis

RNA interference (RNAi) is the method of choice to systematically test for gene function in an intact organism. The model organism Drosophila has the advantage that RNAi is cell autonomous, meaning it does not spread from one cell to the next. Hence, RNAi can be performed in a tissue-specific manner by expressing short or long inverted repeat constructs (hairpins) designed to target mRNAs from one specific target gene. This achieves tissue-specific knock-down of a target gene of choice. Here, we detail the methodology to test gene function in Drosophila muscle tissue by expressing hairpins in a muscle-specific manner using the GAL4-UAS system. We further discuss the systematic RNAi resource collections available which also permit large scale screens in a muscle-specific manner. The full power of such screens is revealed by combination of high-throughput assays followed by detailed morphological assays. Together, this chapter should be a practical guide to enable the reader to either test a few candidate genes, or large gene sets for particular functions in Drosophila muscle tissue and provide first insights into the biological process the gene might be important for in muscle.

Selective Filopodia Adhesion Ensures Robust Cell Matching in the Drosophila Heart

The ability to form specific cell-cell connections within complex cellular environments is critical for multicellular organisms. However, the underlying mechanisms of cell matching that instruct these connections remain elusive. Here, we quantitatively explored the dynamics and regulation of cell matching processes utilizing Drosophila cardiogenesis. We found that cell matching is highly robust at boundaries between cardioblast (CB) subtypes, and filopodia of different CB subtypes have distinct binding affinities. Cdc42 is involved in regulating this selective filopodia binding adhesion and influences CB matching. Further, we identified adhesion molecules Fasciclin III (Fas3) and Ten-m, both of which also regulate synaptic targeting, as having complementary differential expression in CBs. Altering Fas3 expression changes differential filopodia adhesion and leads to CB mismatch. Furthermore, only when both Fas3 and Ten-m are lost is CB alignment severely impaired. Our results show that differential adhesion mediated by selective filopodia binding efficiently regulates precise and robust cell matching.

Insulin signaling modulates border cell movement in Drosophila oogenesis

As collective cell migration is intimately involved in different aspects of metazoan development, molecular mechanisms underlying this process are being explored in a variety of developmental contexts. Border cell (BC) migration during Drosophila oogenesis has emerged as an excellent genetic model for studying collective cell migration. BCs are of epithelial origin but acquire partial mesenchymal characteristics before migrating as a group towards the oocyte. Here, we report that insulin signaling modulates collective BC movement during Drosophila oogenesis. Supporting the involvement of Insulin pathway, we demonstrate that compromising Insulin-like Receptor (InR) levels in BCs, inhibits their migration. Furthermore, we show that canonical Insulin signaling pathway components participate in this process. Interestingly, visualization of InR-depleted BC clusters, using time-lapse imaging, revealed a delay in detachment of BC clusters from the surrounding anterior follicle cells and altered protrusion dynamics. Lastly, based on genetic interactions between InR, the polarity determinant, par-1 and a regulatory subunit of Drosophila Myosin (spaghetti squash), we propose that Insulin signaling likely influences par-1 activity to engineer border cell detachment and subsequent movement via Drosophila Myosin.

Drosophila Embryonic Hemocytes Produce Laminins to Strengthen Migratory Response

The most prominent developmental function attributed to the extracellular matrix (ECM) is cell migration. While cells in culture can produce ECM to migrate, the role of ECM in regulating developmental cell migration is classically viewed as an exogenous matrix presented to the moving cells. In contrast to this view, we show here that Drosophila embryonic hemocytes deposit their own laminins in streak-like structures to migrate efficiently throughout the embryo. With the help of transplantation experiments, live microscopy, and image quantification, we demonstrate that autocrine-produced laminin regulates hemocyte migration by controlling lamellipodia dynamics, stability, and persistence. Proper laminin deposition is regulated by the RabGTPase Rab8, which is highly expressed and required in hemocytes for lamellipodia dynamics and migration. Our results thus support a model in which, during embryogenesis, the Rab8-regulated autocrine deposition of laminin reinforces directional and effective migration by stabilizing cellular protrusions and strengthening otherwise transient adhesion states.

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Drosophila embryonic hemocytes use autocrine-produced laminins for their migration

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Autocrine laminins regulate lamellipodia dynamics, stability, and persistence

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Rab8 regulates laminin deposition and lamellipodia dynamics in migrating hemocytes

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Laminins deposit in tracks around hemocytes and in a fibrillar mesh over the VNC

Fat Body Cells Are Motile and Actively Migrate to Wounds to Drive Repair and Prevent Infection

Adipocytes have many functions in various tissues beyond energy storage, including regulating metabolism, growth, and immunity. However, little is known about their role in wound healing. Here we use live imaging of fat body cells, the equivalent of vertebrate adipocytes in Drosophila, to investigate their potential behaviors and functions following skin wounding. We find that pupal fat body cells are not immotile, as previously presumed, but actively migrate to wounds using an unusual adhesion-independent, actomyosin-driven, peristaltic mode of motility. Once at the wound, fat body cells collaborate with hemocytes, Drosophila macrophages, to clear the wound of cell debris; they also tightly seal the epithelial wound gap and locally release antimicrobial peptides to fight wound infection. Thus, fat body cells are motile cells, enabling them to migrate to wounds to undertake several local functions needed to drive wound repair and prevent infections.

Dynamic analysis of the mesenchymal-epithelial transition of blood-brain barrier forming glia in Drosophila

During development, many epithelia are formed by a mesenchymal-epithelial transition (MET). Here, we examine the major stages and underlying mechanisms of MET during blood-brain barrier formation in Drosophila. We show that contact with the basal lamina is essential for the growth of the barrier-forming subperineurial glia (SPG). Septate junctions (SJs), which provide insulation of the paracellular space, are not required for MET, but are necessary for the establishment of polarized SPG membrane compartments. In vivo time-lapse imaging reveals that the Moody GPCR signaling pathway regulates SPG cell growth and shape, with different levels of signaling causing distinct phenotypes. Timely, well-coordinated SPG growth is essential for the uniform insertion of SJs and thus the insulating function of the barrier. To our knowledge, this is the first dynamic in vivo analysis of all stages in the formation of a secondary epithelium, and of the key role trimeric G protein signaling plays in this important morphogenetic process.

Summary: This study examines the major steps and underlying mechanisms of mesenchymal-epithelial transition of the blood-brain-barrier forming glia in Drosophila, including the role of basal lamina, septate junctions and of trimeric G protein signaling.

Mechanical tension and spontaneous muscle twitching precede the formation of cross-striated muscle in vivo

Muscle forces are produced by repeated stereotypical actomyosin units called sarcomeres. Sarcomeres are chained into linear myofibrils spanning the entire muscle fiber. In mammalian body muscles, myofibrils are aligned laterally, resulting in their typical cross-striated morphology. Despite this detailed textbook knowledge about the adult muscle structure, it is still unclear how cross-striated myofibrils are built in vivo. Here, we investigate the morphogenesis of Drosophila abdominal muscles and establish them as an in vivo model for cross-striated muscle development. By performing live imaging, we find that long immature myofibrils lacking a periodic actomyosin pattern are built simultaneously in the entire muscle fiber and then align laterally to give mature cross-striated myofibrils. Interestingly, laser micro-lesion experiments demonstrate that mechanical tension precedes the formation of the immature myofibrils. Moreover, these immature myofibrils do generate spontaneous Ca²⁺-dependent contractions in vivo, which, when chemically blocked, result in cross-striation defects. Taken together, these results suggest a myofibrillogenesis model in which mechanical tension and spontaneous muscle twitching synchronize the simultaneous self-organization of different sarcomeric protein complexes to build highly regular cross-striated myofibrils spanning the length of large muscle fibers.

Summary: In Drosophila, immature myofibrils are built simultaneously across an entire muscle fiber, and then self-organize in a manner dependent on spontaneous contractions and mechanical tension.

A Gap Junction Protein, Inx2, Modulates Calcium Flux to Specify Border Cell Fate during Drosophila oogenesis

Intercellular communication mediated by gap junction (GJ) proteins is indispensable during embryogenesis, tissue regeneration and wound healing. Here we report functional analysis of a gap junction protein, Innexin 2 (Inx2), in cell type specification during Drosophila oogenesis. Our data reveal a novel involvement of Inx2 in the specification of Border Cells (BCs), a migratory cell type, whose identity is determined by the cell autonomous STAT activity. We show that Inx2 influences BC fate specification by modulating STAT activity via Domeless receptor endocytosis. Furthermore, detailed experimental analysis has uncovered that Inx2 also regulates a calcium flux that transmits across the follicle cells. We propose that Inx2 mediated calcium flux in the follicle cells stimulates endocytosis by altering Dynamin (Shibire) distribution which is in turn critical for careful calibration of STAT activation and, thus for BC specification. Together our data provide unprecedented molecular insights into how gap junction proteins can regulate cell-type specification.

Gap junction mediated intercellular communication modulates several processes during development, morphogenesis and normal tissue homeostasis. While gap junction proteins play an important role during intercellular communication, the underlying molecular mechanism(s) as to how they regulate diverse signaling cascades are unclear. By employing the Drosophila melanogaster oogenesis model we have characterized the role of gap junction protein, Innexin 2 (Inx2), in cell fate specification during Drosophila oogenesis. Our data demonstrate that loss of inx2 affects border cell specification. Border cells are a small group of 6–8 follicle cells that acquire migratory fate in response to the activation of JAK-STAT signaling. We show that perturbing Inx2 levels in the follicle cells inhibits JAK-STAT signaling thereby adversely influencing border cell fate specification. Using live cell imaging and molecular genetic analysis, we have elucidated the molecular mechanism underlying Inx2 function in this process. We show that Inx2 mediates inter-follicular calcium flux that is critical for border cell fate determination. Furthermore, our observations indicate that Inx2 regulates Domeless receptor internalization possibly via influencing distribution of Drosophila Dynamin, Shibire in the follicle cells. Taken together these results suggest a functional link between Inx2, calcium flux and receptor endocytosis during border cell fate specification in Drosophila oogenesis.

piragua encodes a zinc finger protein required for development in Drosophila

We isolated and characterized embryonic lethal mutations in piragua (prg). The prg locus encodes a protein with an amino terminus Zinc Finger-Associated-Domain (ZAD) and nine C₂H₂ zinc fingers (ZF). prg mRNA and protein expression during embryogenesis is dynamic with widespread maternal contribution, and subsequent expression in epithelial precursors. About a quarter of prg mutant embryos do not develop cuticle, and from those that do a small fraction have cuticular defects. Roughly half of prg mutants die during embryogenesis. prg mutants have an extended phenocritical period encompassing embryogenesis and first instar larval stage, since the other half of prg mutants die as first or second instar larvae. During dorsal closure, time-lapse high-resolution imaging shows defects arising out of sluggishness in closure, resolving at times in failures of closure. prg is expressed in imaginal discs, and is required for imaginal development. prg was identified in imaginal tissue in a cell super competition screen, together with other genes, like flower. We find that flower mutations are also embryonic lethal with a similar phenocritical period and strong embryonic mutant phenotypes (head involution defects, primarily). The two loci interact genetically in the embryo, as they increase embryonic mortality to close to 90% with the same embryonic phenotypes (dorsal closure and head involution defects, plus lack of cuticle). Mutant prg clones generated in developing dorsal thorax and eye imaginal tissue have strong developmental defects (lack of bristles and ommatidial malformations). prg is required in several developmental morphogenetic processes.

Imaging Approaches to Investigate Myonuclear Positioning in Drosophila

In the skeletal muscle, nuclei are positioned at the periphery of each myofiber and are evenly distributed along its length. Improper positioning of myonuclei has been correlated with muscle disease and decreased muscle function. Several mechanisms required for regulating nuclear position have been identified using the fruit fly, Drosophila melanogaster. The conservation of the myofiber between the fly and vertebrates, the availability of advanced genetic tools, and the ability to visualize dynamic processes using fluorescent proteins in vivo makes the fly an excellent system to study myonuclear positioning. This chapter describes time-lapse and fixed imaging methodologies using both the Drosophila embryo and the larva to investigate mechanisms of myonuclear positioning.

Pak3 regulates apical-basal polarity in migrating border cells during Drosophila oogenesis

Group cell migration is a highly coordinated process that is involved in a number of physiological events such as morphogenesis, wound healing and tumor metastasis. Unlike single cells, collectively moving cells are physically attached to each other and retain some degree of apical-basal polarity during the migratory phase. Although much is known about direction sensing, how polarity is regulated in multicellular movement remains unclear. Here we report the role of the protein kinase Pak3 in maintaining apical-basal polarity in migrating border cell clusters during Drosophila oogenesis. Pak3 is enriched in border cells and downregulation of its function impedes border cell movement. Time-lapse imaging suggests that Pak3 affects protrusive behavior of the border cell cluster, specifically regulating the stability and directionality of protrusions. Pak3 functions downstream of guidance receptor signaling to regulate the level and distribution of F-actin in migrating border cells. We also provide evidence that Pak3 genetically interacts with the lateral polarity marker Scribble and that it regulates JNK signaling in the moving border cells. Since Pak3 depletion results in mislocalization of several apical-basal polarity markers and overexpression of Jra rescues the polarity of the Pak3-depleted cluster, we propose that Pak3 functions through JNK signaling to modulate apical-basal polarity of the migrating border cell cluster. We also observe loss of apical-basal polarity in Rac1-depleted border cell clusters, suggesting that guidance receptor signaling functions through Rac GTPase and Pak3 to regulate the overall polarity of the cluster and mediate efficient collective movement of the border cells to the oocyte boundary.

Real-time imaging of actin filaments in the zebrafish oocyte and embryo

Dynamic changes of cytoplasmic and cortical actin filaments drive various cellular and developmental processes. Although real-time imaging of actin filaments in living cells has been developed, imaging of actin filaments in specific cells of living organisms remains limited, particularly for the analysis of gamete formation and early embryonic development. Here, we report the production of transgenic zebrafish expressing the C-terminus of Moesin, an actin filament-binding protein, fused with green fluorescent protein or red fluorescent protein (GFP/RFP-MoeC), under the control of a cyclin B1 promoter. GFP/RFP-MoeC was expressed maternally, which labels the cortical actin cytoskeleton of blastula-stage cells. High levels of GFP/RFP fluorescence were detected in the adult ovary and testis. In the ovaries, GFP/RFP-MoeC was expressed in oocytes but not in follicle cells, which allows us to clearly visualize the organization of actin filaments in different stages of the oocyte. Using full-grown oocytes, we revealed the dynamic changes of actin columns assembled in the cortical cytoplasm during oocyte maturation. The number of columns slightly decreased in the early period before germinal vesicle breakdown (GVBD) and then significantly decreased at GVBD, followed by recovery after GVBD. Our transgenic fish are useful for analyzing the dynamics of actin filaments in oogenesis and early embryogenesis.

A role for E-cadherin in ensuring cohesive migration of a heterogeneous population of non-epithelial cells

Collective cell migration is a key process underlying the morphogenesis of many organs as well as tumour invasion, which very often involves heterogeneous cell populations. Here we investigated how such populations can migrate cohesively in the Drosophila posterior midgut, comprised of epithelial and mesenchymal cells and show a novel role for the epithelial adhesion molecule E-cadherin (E-Cad) in mesenchymal cells. Despite a lack of junctions at the ultrastructure level, reducing E-Cad levels causes mesenchymal cells to detach from one another and from neighbouring epithelial cells; as a result, coordination between the two populations is lost. Moreover, Bazooka and recycling mechanisms are also required for E-Cad accumulation in mesenchymal cells. These results indicate an active role for E-Cad in mediating cohesive and ordered migration of non-epithelial cells, and discount the notion of E-Cad as just an epithelial feature that has to be switched off to enable migration of mesenchymal cells.

Endocytosis-dependent coordination of multiple actin regulators is required for wound healing

The ability to heal wounds efficiently is essential for life. After wounding of an epithelium, the cells bordering the wound form dynamic actin protrusions and/or a contractile actomyosin cable, and these actin structures drive wound closure. Despite their importance in wound healing, the molecular mechanisms that regulate the assembly of these actin structures at wound edges are not well understood. In this paper, using Drosophila melanogaster embryos, we demonstrate that Diaphanous, SCAR, and WASp play distinct but overlapping roles in regulating actin assembly during wound healing. Moreover, we show that endocytosis is essential for wound edge actin assembly and wound closure. We identify adherens junctions (AJs) as a key target of endocytosis during wound healing and propose that endocytic remodeling of AJs is required to form "signaling centers" along the wound edge that control actin assembly. We conclude that coordination of actin assembly, AJ remodeling, and membrane traffic is required for the construction of a motile leading edge during wound healing.

PI(4,5)P2 regulates myoblast fusion through Arp2/3 regulator localization at the fusion site

Cell-cell fusion is a regulated process that requires merging of the opposing membranes and underlying cytoskeletons. However, the integration between membrane and cytoskeleton signaling during fusion is not known. Using Drosophila, we demonstrate that the membrane phosphoinositide PI(4,5)P2 is a crucial regulator of F-actin dynamics during myoblast fusion. PI(4,5)P2 is locally enriched and colocalizes spatially and temporally with the F-actin focus that defines the fusion site. PI(4,5)P2 enrichment depends on receptor engagement but is upstream or parallel to actin remodeling. Regulators of actin branching via Arp2/3 colocalize with PI(4,5)P2 in vivo and bind PI(4,5)P2 in vitro. Manipulation of PI(4,5)P2 availability leads to impaired fusion, with a reduction in the F-actin focus size and altered focus morphology. Mechanistically, the changes in the actin focus are due to a failure in the enrichment of actin regulators at the fusion site. Moreover, improper localization of these regulators hinders expansion of the fusion interface. Thus, PI(4,5)P2 enrichment at the fusion site encodes spatial and temporal information that regulates fusion progression through the localization of activators of actin polymerization.

Retrograde BMP signaling modulates rapid activity-dependent synaptic growth via presynaptic LIM kinase regulation of cofilin

The Drosophila neuromuscular junction (NMJ) is capable of rapidly budding new presynaptic varicosities over the course of minutes in response to elevated neuronal activity. Using live imaging of synaptic growth, we characterized this dynamic process and demonstrated that rapid bouton budding requires retrograde bone morphogenic protein (BMP) signaling and local alteration in the presynaptic actin cytoskeleton. BMP acts during development to provide competence for rapid synaptic growth by regulating the levels of the Rho-type guanine nucleotide exchange factor Trio, a transcriptional output of BMP-Smad signaling. In a parallel pathway, we find that the BMP type II receptor Wit signals through the effector protein LIM domain kinase 1 (Limk) to regulate bouton budding. Limk interfaces with structural plasticity by controlling the activity of the actin depolymerizing protein Cofilin. Expression of constitutively active or inactive Cofilin in motor neurons demonstrates that increased Cofilin activity promotes rapid bouton formation in response to elevated synaptic activity. Correspondingly, the overexpression of Limk, which inhibits Cofilin, inhibits bouton budding. Live imaging of the presynaptic F-actin cytoskeleton reveals that activity-dependent bouton addition is accompanied by the formation of new F-actin puncta at sites of synaptic growth. Pharmacological disruption of actin turnover inhibits bouton budding, indicating that local changes in the actin cytoskeleton at pre-existing boutons precede new budding events. We propose that developmental BMP signaling potentiates NMJs for rapid activity-dependent structural plasticity that is achieved by muscle release of retrograde signals that regulate local presynaptic actin cytoskeletal dynamics.

Regulation of branching dynamics by axon-intrinsic asymmetries in Tyrosine Kinase Receptor signaling

Axonal branching allows a neuron to connect to several targets, increasing neuronal circuit complexity. While axonal branching is well described, the mechanisms that control it remain largely unknown. We find that in the Drosophila CNS branches develop through a process of excessive growth followed by pruning. In vivo high-resolution live imaging of developing brains as well as loss and gain of function experiments show that activation of Epidermal Growth Factor Receptor (EGFR) is necessary for branch dynamics and the final branching pattern. Live imaging also reveals that intrinsic asymmetry in EGFR localization regulates the balance between dynamic and static filopodia. Elimination of signaling asymmetry by either loss or gain of EGFR function results in reduced dynamics leading to excessive branch formation. In summary, we propose that the dynamic process of axon branch development is mediated by differential local distribution of signaling receptors.

DOI:http://dx.doi.org/10.7554/eLife.01699.001

In the human brain, 100 billion neurons form 100 trillion connections. Each neuron consists of a cell body with numerous small branch-like projections known as dendrites (from the Greek word for ‘tree’), plus a long cable-like structure called the axon. Neurons receive electrical inputs from neighboring cells via their dendrites, and then relay these signals onto other cells in their network via their axons.

The development of the brain relies on new neurons integrating successfully into existing networks. Axon branching helps with this by enabling a single neuron to establish connections with several cells, but it is unclear how individual neurons decide when and where to form branches. Now, Zschätzsch et al. have revealed the mechanism behind this process in the fruit fly, Drosophila.

Mutant flies that lack a protein called EGFR produce abnormal numbers of axon branches, suggesting that this molecule regulates branch formation. Indeed in fruit flies, just as in mammals, the developing brain initially produces excessive numbers of branches, which are subsequently pruned to leave only those that have formed appropriate connections. In Drosophila, an uneven distribution of EGFR between branches belonging to the same axon acts as a signal to regulate this pruning process.

To examine this mechanism in more detail, high-resolution four-dimensional imaging was used to study brains that had been removed from Drosophila pupae and kept alive in special culture chambers. Axon branching and loss could now be followed in real time, and were found to occur more slowly in brains that lacked EGFR. The receptor controlled the branching of axons by influencing the distribution of another protein called actin, which is a key component of the internal skeleton that gives cells their structure.

In addition to providing new insights into a fundamental aspect of brain development, the work of Zschätzsch et al. also highlights the importance of stochastic events in shaping the network of connections within the developing brain. These findings may well be relevant to ongoing efforts to map the human brain ‘connectome’.

DOI:http://dx.doi.org/10.7554/eLife.01699.002

A guide to study Drosophila muscle biology

The development and molecular composition of muscle tissue is evolutionarily conserved. Drosophila is a powerful in vivo model system to investigate muscle morphogenesis and function. Here, we provide a short and comprehensive overview of the important developmental steps to build Drosophila body muscle in embryos, larvae and pupae. We describe key methods, including muscle histology, live imaging and genetics, to study these steps at various developmental stages and include simple behavioural assays to assess muscle function in larvae and adults. We list valuable antibodies and fly strains that can be used for these different methods. This overview should guide the reader to choose the best marker or the appropriate method to obtain high quality muscle morphogenesis data in Drosophila.

Ecdysone signaling at metamorphosis triggers apoptosis of Drosophila abdominal muscles

One of the most dramatic examples of programmed cell death occurs during Drosophila metamorphosis, when most of the larval tissues are destroyed in a process termed histolysis. Much of our understanding of this process comes from analyses of salivary gland and midgut cell death. In contrast, relatively little is known about the degradation of the larval musculature. Here, we analyze the programmed destruction of the abdominal dorsal exterior oblique muscle (DEOM) which occurs during the first 24h of metamorphosis. We find that ecdysone signaling through Ecdysone receptor isoform B1 is required cell autonomously for the muscle death. Furthermore, we show that the orphan nuclear receptor FTZ-F1, opposed by another nuclear receptor, HR39, plays a critical role in the timing of DEOM histolysis. Finally, we show that unlike the histolysis of salivary gland and midgut, abdominal muscle death occurs by apoptosis, and does not require autophagy. Thus, there is no set rule as to the role of autophagy and apoptosis during Drosophila histolysis.

A dual role for the βPS integrin myospheroid in mediating Drosophila embryonic macrophage migration

Throughout embryonic development, macrophages not only act as the first line of defence against infection but also help to sculpt organs and tissues of the embryo by removing dead cells and secreting extracellular matrix components. Key to their function is the ability of embryonic macrophages to migrate and disperse throughout the embryo. Despite these important developmental functions, little is known about the molecular mechanisms underlying embryonic macrophage migration in vivo. Integrins are key regulators of many of the adult macrophage responses, but their role in embryonic macrophages remains poorly characterized. Here, we have used Drosophila macrophages (haemocytes) as a model system to address the role of integrins during embryonic macrophage dispersal in vivo. We show that the main βPS integrin, myospheroid, affects haemocyte migration in two ways; by shaping the three-dimensional environment in which haemocytes migrate and by regulating the migration of haemocytes themselves. Live imaging revealed a requirement for myospheroid within haemocytes to coordinate the microtubule and actin dynamics, and to enable haemocyte developmental dispersal, contact repulsion and inflammatory migration towards wounds.

Calcium flashes orchestrate the wound inflammatory response through DUOX activation and hydrogen peroxide release

A crucial early wound response is the recruitment of inflammatory cells drawn by danger cues released by the damaged tissue. Hydrogen peroxide (H₂O₂) has recently been identified as the earliest wound attractant in Drosophila embryos and zebrafish larvae [1, 2]. The H₂O₂ signal is generated by activation of an NADPH oxidase, DUOX, and as a consequence, the first inflammatory cells are recruited to the wound within minutes. To date, nothing is known about how wounding activates DUOX. Here, we show that laser wounding of the Drosophila embryo epidermis triggers an instantaneous calcium flash, which travels as a wave via gap junctions several cell rows back from the wound edge. Blocking this calcium flash inhibits H₂O₂ release at the wound site and leads to a reduction in the number of immune cells migrating to the wound. We suggest that the wound-induced calcium flash activates DUOX via an EF hand calcium-binding motif and thus triggers the production of the attractant damage cue H₂O₂. Therefore, calcium represents the earliest signal in the wound inflammatory response.

Par3/Bazooka and phosphoinositides regulate actin protrusion formation during Drosophila dorsal closure and wound healing

Effective wound closure mechanisms are essential for maintenance of epithelial structure and function. The repair of wounded epithelia is primarily driven by the cells bordering the wound, which become motile after wounding, forming dynamic actin protrusions along the wound edge. The molecular mechanisms that trigger wound edge cells to become motile following tissue damage are not well understood. Using wound healing and dorsal closure in Drosophila, we identify a direct molecular link between changes in cell-cell adhesion at epithelial edges and induction of actin protrusion formation. We find that the scaffolding protein Par3/Bazooka and the lipid phosphatase Pten are specifically lost from cell-cell junctions at epithelial edges. This results in a localized accumulation of phosphatidylinositol 3,4,5-trisphosphate (PIP3), which promotes the formation of actin protrusions along the epithelial edge. Depleting PIP3 results in defective epithelial closure during both dorsal closure and wound healing. These data reveal a novel mechanism that directly couples loss of epithelial integrity to activation of epithelial closure.

Emergence of embryonic pattern through contact inhibition of locomotion

The pioneering cell biologist Michael Abercrombie first described the process of contact inhibition of locomotion more than 50 years ago when migrating fibroblasts were observed to rapidly change direction and migrate away upon collision. Since then, we have gleaned little understanding of how contact inhibition is regulated and only lately observed its occurrence in vivo. We recently revealed that Drosophila macrophages (haemocytes) require contact inhibition for their uniform embryonic dispersal. Here, to investigate the role that contact inhibition plays in the patterning of haemocyte movements, we have mathematically analysed and simulated their contact repulsion dynamics. Our data reveal that the final pattern of haemocyte distribution, and the details and timing of its formation, can be explained by contact inhibition dynamics within the geometry of the Drosophila embryo. This has implications for morphogenesis in general as it suggests that patterns can emerge, irrespective of external cues, when cells interact through simple rules of contact repulsion.

Dendritic filopodia, Ripped Pocket, NOMPC, and NMDARs contribute to the sense of touch in Drosophila larvae

BACKGROUND

Among the Aristotelian senses, the subcellular and molecular mechanisms involved in the sense of touch are the most poorly understood.

RESULTS

We demonstrate that specialized sensory neurons, the class II and class III multidendritic (md) neurons, are gentle touch sensors of Drosophila larvae. Genetic silencing of these cells significantly impairs gentle touch responses, optogenetic activation of these cells triggers behavioral touch-like responses, and optical recordings from these neurons show that they respond to force. The class III neurons possess highly dynamic dendritic protrusions rich in F-actin. Genetic manipulations that alter actin dynamics indicate that the actin-rich protrusions (termed sensory filopodia) on the class III neurons are required for behavioral sensitivity to gentle touch. Through a genome-wide RNAi screen of ion channels, we identified Ripped Pocket (rpk), No Mechanoreceptor Potential C (nompC), and NMDA Receptors 1 and 2 (Nmdars) as playing critical roles in both behavioral responses to touch and in the formation of the actin-rich sensory filopodia. Consistent with this requirement, reporters for rpk and nompC show expression in the class III neurons. A genetic null allele of rpk confirms its critical role in touch responses.

CONCLUSIONS

Output from class II and class III md neurons of the Drosophila larvae is necessary and sufficient for eliciting behavioral touch responses. These cells show physiological responses to force. Ion channels in several force-sensing gene families are required for behavioral sensitivity to touch and for the formation of the actin-rich sensory filopodia.

Developmental biology: taking flight

Powered flight was first mastered by insects, many millions of years ago. Now, studies with the fruit fly Drosophila melanogaster reveal the critical role of a conserved transcription factor in programming the development of specialized flight muscles.

Fascin promotes filopodia formation independent of its role in actin bundling

Mutation of a critical residue of fascin eliminates the protein’s actin-bundling activity but maintains its positive role in filopodia formation

Fascin is an evolutionarily conserved actin-binding protein that plays a key role in forming filopodia. It is widely thought that this function involves fascin directly bundling actin filaments, which is controlled by an N-terminal regulatory serine residue. In this paper, by studying cellular processes in Drosophila melanogaster that require fascin activity, we identify a regulatory residue within the C-terminal region of the protein (S289). Unexpectedly, although mutation (S289A) of this residue disrupted the actin-bundling capacity of fascin, fascin S289A fully rescued filopodia formation in fascin mutant flies. Live imaging of migrating macrophages in vivo revealed that this mutation restricted the localization of fascin to the distal ends of filopodia. The corresponding mutation of human fascin (S274) similarly affected its interaction with actin and altered filopodia dynamics within carcinoma cells. These data reveal an evolutionarily conserved role for this regulatory region and unveil a function for fascin, uncoupled from actin bundling, at the distal end of filopodia.

The RhoGEF trio functions in sculpting class specific dendrite morphogenesis in Drosophila sensory neurons

Background

As the primary sites of synaptic or sensory input in the nervous system, dendrites play an essential role in processing neuronal and sensory information. Moreover, the specification of class specific dendrite arborization is critically important in establishing neural connectivity and the formation of functional networks. Cytoskeletal modulation provides a key mechanism for establishing, as well as reorganizing, dendritic morphology among distinct neuronal subtypes. While previous studies have established differential roles for the small GTPases Rac and Rho in mediating dendrite morphogenesis, little is known regarding the direct regulators of these genes in mediating distinct dendritic architectures.

Methodology/Principal Findings

Here we demonstrate that the RhoGEF Trio is required for the specification of class specific dendritic morphology in dendritic arborization (da) sensory neurons of the Drosophila peripheral nervous system (PNS). Trio is expressed in all da neuron subclasses and loss-of-function analyses indicate that Trio functions cell-autonomously in promoting dendritic branching, field coverage, and refining dendritic outgrowth in various da neuron subtypes. Moreover, overexpression studies demonstrate that Trio acts to promote higher order dendritic branching, including the formation of dendritic filopodia, through Trio GEF1-dependent interactions with Rac1, whereas Trio GEF-2-dependent interactions with Rho1 serve to restrict dendritic extension and higher order branching in da neurons. Finally, we show that de novo dendritic branching, induced by the homeodomain transcription factor Cut, requires Trio activity suggesting these molecules may act in a pathway to mediate dendrite morphogenesis.

Conclusions/Significance

Collectively, our analyses implicate Trio as an important regulator of class specific da neuron dendrite morphogenesis via interactions with Rac1 and Rho1 and indicate that Trio is required as downstream effector in Cut-mediated regulation of dendrite branching and filopodia formation.

In vivo imaging of the actin polymerization state with two-photon fluorescence anisotropy

Using two-photon fluorescence anisotropy imaging of actin-GFP, we have developed a method for imaging the actin polymerization state that is applicable to a broad range of experimental systems extending from fixed cells to live animals. The incorporation of expressed actin-GFP monomers into endogenous actin polymers enables energy migration FRET (emFRET, or homoFRET) between neighboring actin-GFPs. This energy migration reduces the normally high polarization of the GFP fluorescence. We derive a simple relationship between the actin-GFP fluorescence polarization anisotropy and the actin polymer fraction, thereby enabling a robust means of imaging the actin polymerization state with high spatiotemporal resolution and providing what to the best of our knowledge are the first direct images of the actin polymerization state in live, adult brain tissue and live, intact Drosophila larvae.

Microfluidic chips for in vivo imaging of cellular responses to neural injury in Drosophila larvae

With powerful genetics and a translucent cuticle, the Drosophila larva is an ideal model system for live imaging studies of neuronal cell biology and function. Here, we present an easy-to-use approach for high resolution live imaging in Drosophila using microfluidic chips. Two different designs allow for non-invasive and chemical-free immobilization of 3^rd instar larvae over short (up to 1 hour) and long (up to 10 hours) time periods. We utilized these ‘larva chips’ to characterize several sub-cellular responses to axotomy which occur over a range of time scales in intact, unanaesthetized animals. These include waves of calcium which are induced within seconds of axotomy, and the intracellular transport of vesicles whose rate and flux within axons changes dramatically within 3 hours of axotomy. Axonal transport halts throughout the entire distal stump, but increases in the proximal stump. These responses precede the degeneration of the distal stump and regenerative sprouting of the proximal stump, which is initiated after a 7 hour period of dormancy and is associated with a dramatic increase in F-actin dynamics. In addition to allowing for the study of axonal regeneration in vivo, the larva chips can be utilized for a wide variety of in vivo imaging applications in Drosophila.

Pathogenic polyglutamine proteins cause dendrite defects associated with specific actin cytoskeletal alterations in Drosophila

Whereas the neurodegeneration associated with various polyglutamine (polyQ) diseases has prompted extensive studies of polyQ-induced cell death, the neuronal loss that typically appears during late stages of the diseases may not account for the preceding movement and mental disorders. The cellular basis for polyQ-induced neuronal dysfunction preceding neuronal cell death remains largely unknown. Here we report defective dendrite morphogenesis within a specific subset of neurons due to polyQ toxicity that can be dissociated from caspase-dependent cell death. Expressing pathogenic spinocerebellar ataxia type 1 (SCA1) or type 3 (SCA3) proteins in Drosophila larval dendritic arborization neurons caused neuronal type-specific dendrite phenotypes primarily affecting terminal branches. We further show that expression of pathogenic polyQ proteins in adult flies after the formation of neuronal dendrites also greatly reduced dendritic complexity. These defects are associated with disruption of dendritic F-actin structures that can be partially mitigated by increasing Rac-PAK signaling. Together, these findings suggest that specific actin cytoskeletal alterations that alter dendrite morphology and function may contribute to the pathogenesis of at least a subset of polyQ disorders, including SCA3 and SCA1.

The WASp-based actin polymerization machinery is required in somatic support cells for spermatid maturation and release

WASp family proteins serve as conserved regulators of branched microfilament array formation via the Arp2/3 actin polymerization machinery. We have identified a specific role during spermatogenesis for the Drosophila WASp homolog (Wsp) and associated elements. Spermatogenesis within the fly testis is carried out in cysts, where a pair of somatic cyst cells encloses differentiating sperm. The final phase of the process involves the attachment of matured cysts to a specialized epithelium at the base of the testis, followed by release of individual motile spermatids into the adjoining seminal vesicle. Wsp mutant cysts contain fully mature sperm, but spermatid release does not occur, resulting in male sterility. Our data suggest that the Wsp-Arp2/3-based machinery acts in the cyst cells to influence proper microfilament organization and to enable cyst attachment to the base of the testis. Wsp activity in this context is mediated by the small GTPase Cdc42. Involvement of the cell surface protein Sticks and stones and the Wsp adapter protein D-WIP (Vrp1) is also crucial. In parallel, we demonstrate that N-WASp (Wasl), the major mammalian WASp family protein, is required in the somatic Sertoli cells of the mouse testis for sperm maturation. A requirement for WASp-based activity in somatic support cells therefore appears to be a universal feature of spermatogenesis.

Recent advances in imaging embryonic myoblast fusion in Drosophila

Myoblast fusion in the Drosophila embryos is a complex process that includes changes in cell movement, morphology and behavior over time. The advent of fluorescent proteins (FPs) has made it possible to track and image live cells, to capture the process of myoblast fusion, and to carry out quantitative analysis of myoblasts in real time. By tagging proteins with FPs, it is also possible to monitor the subcellular events that accompany the fusion process. Herein, we discuss the recent progress that has been made in imaging myoblast fusion in Drosophila, reagents that are now available, and microscopy conditions to consider. Using an Actin-FP fusion protein along with a membrane marker to outline the cells, we show the dynamic formation and breakdown of F-actin foci at sites of fusion. We also describe the methods used successfully to show that these foci are primarily if not wholly present in the fusion-competent myoblasts.

The actin nucleator WASp is required for myoblast fusion during adult Drosophila myogenesis

Myoblast fusion provides a fundamental, conserved mechanism for muscle fiber growth. We demonstrate here that the functional contribution of Wsp, the Drosophila homolog of the conserved actin nucleation-promoting factor (NPF) WASp, is essential for myoblast fusion during the formation of muscles of the adult fly. Disruption of Wsp function results in complete arrest of myoblast fusion in all muscles examined. Wsp activity during adult Drosophila myogenesis is specifically required for muscle cell fusion and is crucial both for the formation of new muscle fibers and for the growth of muscles derived from persistent larval templates. Although Wsp is expressed both in fibers and individual myoblasts, its activity in either one of these cell types is sufficient. SCAR, a second major Arp2/3 NPF, is also required during adult myoblast fusion. Formation of fusion-associated actin 'foci' is dependent on Arp2/3 complex function, but appears to rely on a distinct, unknown nucleator. The comprehensive nature of these requirements identifies Arp2/3-based branched actin polymerization as a universal mechanism underlying myoblast fusion.

Live imaging of the Drosophila spermatogonial stem cell niche reveals novel mechanisms regulating germline stem cell output

Adult stem cells modulate their output by varying between symmetric and asymmetric divisions, but have rarely been observed in living intact tissues. Germline stem cells (GSCs) in the Drosophila testis are anchored to somatic hub cells and were thought to exclusively undergo oriented asymmetric divisions, producing one stem cell that remains hub-anchored and one daughter cell displaced out of the stem cell-maintaining micro-environment (niche). We developed extended live imaging of the Drosophila testis niche, allowing us to track individual germline cells. Surprisingly, new wild-type GSCs are generated in the niche during steady-state tissue maintenance by a previously undetected event we term 'symmetric renewal', where interconnected GSC-daughter cell pairs swivel such that both cells contact the hub. We also captured GSCs undergoing direct differentiation by detaching from the hub. Following starvation-induced GSC loss, GSC numbers are restored by symmetric renewals. Furthermore, upon more severe (genetically induced) GSC loss, both symmetric renewal and de-differentiation (where interconnected spermatogonia fragment into pairs while moving towards then establishing contact with the hub) occur simultaneously to replenish the GSC pool. Thus, stereotypically oriented stem cell divisions are not always correlated with an asymmetric outcome in cell fate, and changes in stem cell output are governed by altered signals in response to tissue requirements.

Hts/Adducin controls synaptic elaboration and elimination

Neural development requires both synapse elaboration and elimination, yet relatively little is known about how these opposing activities are coordinated. Here, we provide evidence Hts/Adducin can serve this function. We show that Drosophila Hts/Adducin is enriched both pre- and postsynaptically at the NMJ. We then demonstrate that presynaptic Hts/Adducin is necessary and sufficient to control two opposing processes associated with synapse remodeling: (1) synapse stabilization as determined by light level and ultrastructural and electrophysiological assays and (2) the elaboration of actin-based, filopodia-like protrusions that drive synaptogenesis and growth. Synapse remodeling is sensitive to Hts/Adducin levels, and we provide evidence that the synaptic localization of Hts/Adducin is controlled via phosphorylation. Mechanistically, Drosophila Hts/Adducin protein has actin-capping activity. We propose that phosphorylation-dependent regulation of Hts/Adducin controls the level, localization, and activity of Hts/Adducin, influencing actin-based synapse elaboration and spectrin-based synapse stabilization. Hts/Adducin may define a mechanism to switch between synapse stability and dynamics.

Differential regulation of dendritic and axonal development by the novel Krüppel-like factor Dar1

Dendrites and axons are two major neuronal compartments with differences that are critical for neuronal functions. To learn about the differential regulation of dendritic and axonal development, we conducted a genetic screen in Drosophila and isolated the dendritic arbor reduction 1 (dar1) mutants, which display defects in dendritic but not axonal growth. The dar1 gene encodes a novel transcription regulator in the Krüppel-like factor family. Neurons lacking dar1 function have severely reduced growth of microtubule- but not F-actin-based dendritic branches. In contrast, overexpression of Dar1 dramatically increased the growth of microtubule-based dendritic branches. Our results suggest that Dar1 promotes dendrite growth in part by suppressing the expression of the microtubule-severing protein Spastin. Our study thus uncovers a novel transcriptional program for microtubule regulation that preferentially controls dendrite growth.

Neuronal remodeling and apoptosis require VCP-dependent degradation of the apoptosis inhibitor DIAP1

The regulated degeneration of axons or dendrites (pruning) and neuronal apoptosis are widely used during development to determine the specificity of neuronal connections. Pruning and apoptosis often share similar mechanisms; for example, developmental dendrite pruning of Drosophila class IV dendritic arborization (da) neurons is induced by local caspase activation triggered by ubiquitin-mediated degradation of the caspase inhibitor DIAP1. Here, we examined the function of Valosin-containing protein (VCP), a ubiquitin-selective AAA chaperone involved in endoplasmic reticulum-associated degradation, autophagy and neurodegenerative disease, in Drosophila da neurons. Strong VCP inhibition is cell lethal, but milder inhibition interferes with dendrite pruning and developmental apoptosis. These defects are associated with impaired caspase activation and high DIAP1 levels. In cultured cells, VCP binds to DIAP1 in a ubiquitin- and BIR domain-dependent manner and facilitates its degradation. Our results establish a new link between ubiquitin, dendrite pruning and the apoptosis machinery.

Imaging pluripotent cell migration in Drosophila

Drosophila melanogaster offers a powerful system for the analysis of cell migration. In the embryo, -pluripotent cells of the mesodermal and endodermal primordia undergo epithelial-mesenchymal transitions and cell migration, while primordial germ cells migrate through an endodermal barrier to form the gonads. Visualisation of these migrations has traditionally been achieved by staining fixed embryos at different developmental stages or through live imaging of cells using tissue-specific expression of marker fluorescent proteins. More recently, photoactivatable fluorescence proteins have allowed the labelling of small groups of cells or single cells so that their migratory patterns and fate can be followed. By fusing the photoactivatable fluorescent protein to proteins that mark different subcellular components, it is now possible to visualise different aspects of the cells as they migrate. Here, we review previous studies of the migration of pluripotent embryonic cells and describe, in detail, methods for visualising these cells.

Clasp-mediated microtubule bundling regulates persistent motility and contact repulsion in Drosophila macrophages in vivo

Drosophila melanogaster macrophages are highly migratory cells that lend themselves beautifully to high resolution in vivo imaging experiments. By expressing fluorescent probes to reveal actin and microtubules, we can observe the dynamic interplay of these two cytoskeletal networks as macrophages migrate and interact with one another within a living organism. We show that before an episode of persistent motility, whether responding to developmental guidance or wound cues, macrophages assemble a polarized array of microtubules that bundle into a compass-like arm that appears to anticipate the direction of migration. Whenever cells collide with one another, their microtubule arms transiently align just before cell-cell repulsion, and we show that forcing depolymerization of microtubules by expression of Spastin leads to their defective polarity and failure to contact inhibit from one another. The same is true in orbit/clasp mutants, indicating a pivotal role for this microtubule-binding protein in the assembly and/or functioning of the microtubule arm during polarized migration and contact repulsion.

pHMA, a pH-sensitive GFP reporter for cell engulfment, in Drosophila embryos, tissues, and cells

Engulfment of apoptotic cells by phagocytosis ensures the removal of unwanted and defective cells. We developed a genetically encoded marker for cell engulfment, pHMA, which consists of the pH-Sensitive derivative of GFP, pHluorin, fused to the actin-binding domain of Moesin. In healthy cells of Drosophila embryos and cultured cells, pHMA resides at the cell cortex. In dying cells, pHMA loses its cortical localization and reports a modest decrease in pH. In embryos, the dying cells lose their apical contacts, then move basally and are ultimately engulfed by neighboring cells or macrophages. The cell corpse material is strongly acidified soon after engulfment and persists in the phagocytic cell for several hours. Changes in the pHMA signal correlate well with increases or decreases in apoptosis. These data show that pHMA is a useful reporter for cell engulfment and can be used in screening for mutations that affect cell engulfment.

Interdependence of macrophage migration and ventral nerve cord development in Drosophila embryos

During embryonic development, Drosophila macrophages (haemocytes) undergo a series of stereotypical migrations to disperse throughout the embryo. One major migratory route is along the ventral nerve cord (VNC), where haemocytes are required for the correct development of this tissue. We show, for the first time, that a reciprocal relationship exists between haemocytes and the VNC and that defects in nerve cord development prevent haemocyte migration along this structure. Using live imaging, we demonstrate that the axonal guidance cue Slit and its receptor Robo are both required for haemocyte migration, but signalling is not autonomously required in haemocytes. We show that the failure of haemocyte migration along the VNC in slit mutants is not due to a lack of chemotactic signals within this structure, but rather to a failure in its detachment from the overlying epithelium, creating a physical barrier to haemocyte migration. This block of haemocyte migration in turn disrupts the formation of the dorsoventral channels within the VNC, further highlighting the importance of haemocyte migration for correct neural development. This study illustrates the important role played by the three-dimensional environment in directing cell migration in vivo and reveals an intriguing interplay between the developing nervous system and the blood cells within the fly, demonstrating that their development is both closely coupled and interdependent.

Live imaging of Drosophila melanogaster embryonic hemocyte migrations

Many studies address cell migration using in vitro methods, whereas the physiologically relevant environment is that of the organism itself. Here we present a protocol for the mounting of Drosophila melanogaster embryos and subsequent live imaging of fluorescently labeled hemocytes, the embryonic macrophages of this organism. Using the Gal4-uas system¹ we drive the expression of a variety of genetically encoded, fluorescently tagged markers in hemocytes to follow their developmental dispersal throughout the embryo. Following collection of embryos at the desired stage of development, the outer chorion is removed and the embryos are then mounted in halocarbon oil between a hydrophobic, gas-permeable membrane and a glass coverslip for live imaging. In addition to gross migratory parameters such as speed and directionality, higher resolution imaging coupled with the use of fluorescent reporters of F-actin and microtubules can provide more detailed information concerning the dynamics of these cytoskeletal components.

Dedifferentiating spermatogonia outcompete somatic stem cells for niche occupancy in the Drosophila testis

Differentiating cells can dedifferentiate to replace stem cells in aged or damaged tissues, but the underlying mechanisms are unknown. In the Drosophila testis, a cluster of stromal cells called the hub creates a niche by locally activating Janus kinase-signal transducer and activator of transcription (Jak-STAT) signaling in adjacent germline and somatic stem cells. Here, we establish a system to study spermatogonial dedifferentiation. Ectopically expressing the differentiation factor bag-of-marbles (Bam) removes germline stem cells from the niche. However, withdrawing ectopic Bam causes interconnected spermatogonia to fragment, move into the niche, exchange positions with resident somatic stem cells, and establish contact with the hub. Concomitantly, actin-based protrusions appear on subsets of spermatogonia, suggesting acquired motility. Furthermore, global downregulation of Jak-STAT signaling inhibits dedifferentiation, indicating that normal levels of pathway activation are required to promote movement of spermatogonia into the niche during dedifferentiation, where they outcompete somatic stem cells for niche occupancy.

Fascin is required for blood cell migration during Drosophilaembryogenesis

Fascin is well characterized in vitro as an actin-bundling protein and its increased expression is correlated with the invasiveness of various cancers. However, the actual roles and regulation of Fascin in vivo remain elusive. Here we show that Fascin is required for the invasive-like migration of blood cells in Drosophila embryos. Fascin expression is highly regulated during embryonic development and, within the blood lineage, is specific to the motile subpopulation of cells, which comprises macrophage-like plasmatocytes. We show that Fascin is required for plasmatocyte migration, both as these cells undergo developmental dispersal and during an inflammatory response to epithelial wounding. Live analyses further demonstrate that Fascin localizes to, and is essential for the assembly of, dynamic actin-rich microspikes within plasmatocyte lamellae that polarize towards the direction of migration. We show that a regulatory serine of Fascin identified from in vitro studies is not required for in vivo cell motility, but is crucial for the formation of actin bundles within epithelial bristles. Together, these results offer a first glimpse into the mechanisms regulating Fascin function during normal development, which might be relevant for understanding the impact of Fascin in cancers.