Fusion pore dynamics of large secretory vesicles define a distinct mechanism of exocytosis

Exocrine cells utilize large secretory vesicles (LSVs) up to 10 μm in diameter. LSVs fuse with the apical surface, often recruiting actomyosin to extrude their content through dynamic fusion pores. The molecular mechanism regulating pore dynamics remains largely uncharacterized. We observe that the fusion pores of LSVs in the Drosophila larval salivary glands expand, stabilize, and constrict. Arp2/3 is essential for pore expansion and stabilization, while myosin II is essential for pore constriction. We identify several Bin-Amphiphysin-Rvs (BAR) homology domain proteins that regulate fusion pore expansion and stabilization. We show that the I-BAR protein Missing-in-Metastasis (MIM) localizes to the fusion site and is essential for pore expansion and stabilization. The MIM I-BAR domain is essential but not sufficient for localization and function. We conclude that MIM acts in concert with actin, myosin II, and additional BAR-domain proteins to control fusion pore dynamics, mediating a distinct mode of exocytosis, which facilitates actomyosin-dependent content release that maintains apical membrane homeostasis during secretion.

ERK1/2 inhibition promotes robust myotube growth via CaMKII activation resulting in myoblast-to-myotube fusion

Myoblast fusion is essential for muscle development and regeneration. Yet, it remains poorly understood how mononucleated myoblasts fuse with preexisting fibers. We demonstrate that ERK1/2 inhibition (ERKi) induces robust differentiation and fusion of primary mouse myoblasts through a linear pathway involving RXR, ryanodine receptors, and calcium-dependent activation of CaMKII in nascent myotubes. CaMKII activation results in myotube growth via fusion with mononucleated myoblasts at a fusogenic synapse. Mechanistically, CaMKII interacts with and regulates MYMK and Rac1, and CaMKIIδ/γ knockout mice exhibit smaller regenerated myofibers following injury. In addition, the expression of a dominant negative CaMKII inhibits the formation of large multinucleated myotubes. Finally, we demonstrate the evolutionary conservation of the pathway in chicken myoblasts. We conclude that ERK1/2 represses a signaling cascade leading to CaMKII-mediated fusion of myoblasts to myotubes, providing an attractive target for the cultivated meat industry and regenerative medicine.

Generation and timing of graded responses to morphogen gradients

Morphogen gradients are known to subdivide a naive cell field into distinct zones of gene expression. Here, we examine whether morphogens can also induce a graded response within such domains. To this end, we explore the role of the Dorsal protein nuclear gradient along the dorsoventral axis in defining the graded pattern of actomyosin constriction that initiates gastrulation in early Drosophila embryos. Two complementary mechanisms for graded accumulation of mRNAs of crucial zygotic Dorsal target genes were identified. First, activation of target-gene expression expands over time from the ventral-most region of high nuclear Dorsal to lateral regions, where the levels are lower, as a result of a Dorsal-dependent activation probability of transcription sites. Thus, sites that are activated earlier will exhibit more mRNA accumulation. Second, once the sites are activated, the rate of RNA Polymerase II loading is also dependent on Dorsal levels. Morphological restrictions require that translation of the graded mRNA be delayed until completion of embryonic cell formation. Such timing is achieved by large introns, which provide a delay in production of the mature mRNAs. Spatio-temporal regulation of key zygotic genes therefore shapes the pattern of gastrulation.

Exocytosis by vesicle crumpling maintains apical membrane homeostasis during exocrine secretion

Exocrine secretion commonly employs micron-scale vesicles that fuse to a limited apical surface, presenting an extreme challenge for maintaining membrane homeostasis. Using Drosophila melanogaster larval salivary glands, we show that the membranes of fused vesicles undergo actomyosin-mediated folding and retention, which prevents them from incorporating into the apical surface. In addition, the diffusion of proteins and lipids between the fused vesicle and the apical surface is limited. Actomyosin contraction and membrane crumpling are essential for recruiting clathrin-mediated endocytosis to clear the retained vesicular membrane. Finally, we also observe membrane crumpling in secretory vesicles of the mouse exocrine pancreas. We conclude that membrane sequestration by crumpling followed by targeted endocytosis of the vesicular membrane, represents a general mechanism of exocytosis that maintains membrane homeostasis in exocrine tissues that employ large secretory vesicles.

Microtubules provide guidance cues for myofibril and sarcomere assembly and growth

BACKGROUND

Muscle myofibrils and sarcomeres present exceptional examples of highly ordered cytoskeletal filament arrays, whose distinct spatial organization is an essential aspect of muscle cell functionality. We utilized ultra-structural analysis to investigate the assembly of myofibrils and sarcomeres within developing myotubes of the indirect flight musculature of Drosophila.

RESULTS

A temporal sequence composed of three major processes was identified: subdivision of the unorganized cytoplasm of nascent, multi-nucleated myotubes into distinct organelle-rich and filament-rich domains; initial organization of the filament-rich domains into myofibrils harboring nascent sarcomeric units; and finally, maturation of the highly-ordered pattern of sarcomeric thick (myosin-based) and thin (microfilament-based) filament arrays in parallel to myofibril radial growth. Significantly, organized microtubule arrays were present throughout these stages and exhibited dynamic changes in their spatial patterns consistent with instructive roles. Genetic manipulations confirm these notions, and imply specific and critical guidance activities of the microtubule-based cytoskeleton, as well as structural interdependence between the myosin- and actin-based filament arrays.

CONCLUSIONS

Our observations highlight a surprisingly significant, behind-the-scenes role for microtubules in establishment of myofibril and sarcomere spatial patterns and size, and provide a detailed account of the interplay between major cytoskeletal elements in generating these essential contractile myogenic units.

Dynamics of Spaetzle morphogen shuttling in the Drosophila embryo shapes gastrulation patterning

Establishment of morphogen gradients in the early Drosophila embryo is challenged by a diffusible sextracellular milieu, and by rapid nuclear divisions that occur at the same time. To understand how a sharp gradient is formed within this dynamic environment, we followed the generation of graded nuclear Dorsal protein, the hallmark of pattern formation along the dorso-ventral axis, in live embryos. The dynamics indicate that a sharp extracellular gradient is formed through diffusion-based shuttling of the Spaetzle (Spz) morphogen that progresses through several nuclear divisions. Perturbed shuttling in wntD mutant embryos results in a flat activation peak and aberrant gastrulation. Re-entry of Dorsal into the nuclei at the final division cycle plays an instructive role, as the residence time of Dorsal in each nucleus is translated to the amount of zygotic transcript that will be produced, thereby guiding graded accumulation of specific zygotic transcripts that drive patterned gastrulation. We conclude that diffusion-based ligand shuttling, coupled with dynamic readout, establishes a refined pattern within the diffusible environment of early embryos.

Feedback inhibition of actin on Rho mediates content release from large secretory vesicles

This work identified a cycle of actin assembly and disassembly in large secretory vesicles of Drosophila salivary glands. Actin disassembly is triggered by actin-dependent recruitment of a RhoGAP protein and is essential for the contractility of the vesicle, leading to content release to the lumen.

Secretion of adhesive glycoproteins to the lumen of Drosophila melanogaster larval salivary glands is performed by contraction of an actomyosin network assembled around large secretory vesicles, after their fusion to the apical membranes. We have identified a cycle of actin coat nucleation and disassembly that is independent of myosin. Recruitment of active Rho1 to the fused vesicle triggers activation of the formin Diaphanous and actin nucleation. This leads to actin-dependent localization of a RhoGAP protein that locally shuts off Rho1, promoting disassembly of the actin coat. When contraction of vesicles is blocked, the strict temporal order of the recruited elements generates repeated oscillations of actin coat formation and disassembly. Interestingly, different blocks to actin coat disassembly arrested vesicle contraction, indicating that actin turnover is an integral part of the actomyosin contraction cycle. The capacity of F-actin to trigger a negative feedback on its own production may be widely used to coordinate a succession of morphogenetic events or maintain homeostasis.

The Drosophila formin Fhos is a primary mediator of sarcomeric thin-filament array assembly

Actin-based thin filament arrays constitute a fundamental core component of muscle sarcomeres. We have used formation of the Drosophila indirect flight musculature for studying the assembly and maturation of thin-filament arrays in a skeletal muscle model system. Employing GFP-tagged actin monomer incorporation, we identify several distinct phases in the dynamic construction of thin-filament arrays. This sequence includes assembly of nascent arrays after an initial period of intensive microfilament synthesis, followed by array elongation, primarily from filament pointed-ends, radial growth of the arrays via recruitment of peripheral filaments and continuous barbed-end turnover. Using genetic approaches we have identified Fhos, the single Drosophila homolog of the FHOD sub-family of formins, as a primary and versatile mediator of IFM thin-filament organization. Localization of Fhos to the barbed-ends of the arrays, achieved via a novel N-terminal domain, appears to be a critical aspect of its sarcomeric roles.

DOI:http://dx.doi.org/10.7554/eLife.16540.001

Muscles owe their ability to contract to structural units called sarcomeres, and a single muscle fiber can contain many thousands of these structures, aligned one next to the other. Each mature sarcomere is made up of precisely arranged and intertwined thin filaments of actin and thicker bundles of motor proteins, surrounded by other proteins. Sliding the motors along the filaments provides the force needed to contract the muscle. However, it was far from clear how sarcomeres, especially the arrays of thin-filaments, are assembled from scratch in developing muscles.

When the fruit fly Drosophila transforms from a larva into an adult, it needs to build muscles to move its newly forming wings. While smaller in size, these flight muscles closely resemble the skeletal muscles of animals with backbones, and therefore serve as a good model for muscle formation in general. New muscles require new sarcomeres too, and now Shwartz et al. have observed and monitored sarcomeres assembling in developing flight muscles of fruit flies, a process that takes about three days.

The analysis made use of genetically engineered flies in which the gene for a fluorescently labeled version of actin, the building block of the thin filaments, could be switched on at specific points in time. Looking at how these green-glowing proteins become incorporated into the growing sarcomere revealed that the assembly process involves four different phases. First, a large store of unorganized and newly-made thin filaments is generated for future use. These filaments are then assembled into rudimentary structures in which the filaments are roughly aligned. Once these core structures are formed, the existing filaments are elongated, while additional filaments are brought in to expand the structure further. Finally, actin proteins are continuously added and removed at the part of the sarcomere where the thin filaments are anchored.

Shwartz et al. went on to identify a protein termed Fhos as the chief player in the process. Fhos is a member of a family of proteins that are known to elongate and organize actin filaments in many different settings. Without Fhos, the thin-filament arrays cannot properly begin to assemble, and the subsequent steps of growth and expansion are blocked as well.

The next challenges will be to understand what guides the initial stages in the assembly of the thin-filament array, and how the coordination between assembly of actin filament arrays and motor proteins is executed. It will also be important to determine how sarcomeres are maintained throughout the life of the organism when defective actin filaments are replaced, and which proteins are responsible for carrying out this process.

DOI:http://dx.doi.org/10.7554/eLife.16540.002

A WntD-Dependent Integral Feedback Loop Attenuates Variability in Drosophila Toll Signaling

Patterning by morphogen gradients relies on the capacity to generate reproducible distribution profiles. Morphogen spread depends on kinetic parameters, including diffusion and degradation rates, which vary between embryos, raising the question of how variability is controlled. We examined this in the context of Toll-dependent dorsoventral (DV) patterning of the Drosophila embryo. We find that low embryo-to-embryo variability in DV patterning relies on wntD, a Toll-target gene expressed initially at the posterior pole. WntD protein is secreted and disperses in the extracellular milieu, associates with its receptor Frizzled4, and inhibits the Toll pathway by blocking the Toll extracellular domain. Mathematical modeling predicts that WntD accumulates until the Toll gradient narrows to its desired spread, and we support this feedback experimentally. This circuit exemplifies a broadly applicable induction-contraction mechanism, which reduces patterning variability through a restricted morphogen-dependent expression of a secreted diffusible inhibitor.

Surface apposition and multiple cell contacts promote myoblast fusion in Drosophila flight muscles

Transmission EM methods reveal that cell–cell fusion of individual myoblasts with growing Drosophila flight muscles is a stepwise process in which the cell adhesion and branched actin machineries mediate tight apposition and formation of multiple contacts and pores between the surfaces of the fusing cells.

Fusion of individual myoblasts to form multinucleated myofibers constitutes a widely conserved program for growth of the somatic musculature. We have used electron microscopy methods to study this key form of cell–cell fusion during development of the indirect flight muscles (IFMs) of Drosophila melanogaster. We find that IFM myoblast–myotube fusion proceeds in a stepwise fashion and is governed by apparent cross talk between transmembrane and cytoskeletal elements. Our analysis suggests that cell adhesion is necessary for bringing myoblasts to within a minimal distance from the myotubes. The branched actin polymerization machinery acts subsequently to promote tight apposition between the surfaces of the two cell types and formation of multiple sites of cell–cell contact, giving rise to nascent fusion pores whose expansion establishes full cytoplasmic continuity. Given the conserved features of IFM myogenesis, this sequence of cell interactions and membrane events and the mechanistic significance of cell adhesion elements and the actin-based cytoskeleton are likely to represent general principles of the myoblast fusion process.

The Edges of Pancreatic Islet β Cells Constitute Adhesive and Signaling Microdomains

Pancreatic islet β cells are organized in rosette-like structures around blood vessels and exhibit an artery-to-vein orientation, but they do not display the typical epithelial polarity. It is unclear whether these cells present a functional asymmetry related to their spatial organization. Here, we identify murine β cell edges, the sites at which adjacent cell faces meet at a sharp angle, as surface microdomains of cell-cell adhesion and signaling. The edges are marked by enrichment of F-actin and E-cadherin and are aligned between neighboring cells. The edge organization is E-cadherin contact dependent and correlates with insulin secretion capacity. Edges display elevated levels of glucose transporters and SNAP25 and extend numerous F-actin-rich filopodia. A similar β cell edge organization was observed in human islets. When stimulated, β cell edges exhibit high calcium levels. In view of the functional importance of intra-islet communication, the spatial architecture of their edges may prove fundamental for coordinating physiological insulin secretion.

Assessing the secretory capacity of pancreatic acinar cells

Pancreatic acinar cells produce and secrete digestive enzymes. These cells are organized as a cluster which forms and shares a joint lumen. This work demonstrates how the secretory capacity of these cells can be assessed by culture of isolated acini. The setup is advantageous since isolated acini, which retain many characteristics of the intact exocrine pancreas can be manipulated and monitored more readily than in the whole animal. Proper isolation of pancreatic acini is a key requirement so that the ex vivo culture will represent the in vivo nature of the acini. The protocol demonstrates how to isolate intact acini from the mouse pancreas. Subsequently, two complementary methods for evaluating pancreatic secretion are presented. The amylase secretion assay serves as a global measure, while direct imaging of pancreatic secretion allows the characterization of secretion at a sub-cellular resolution. Collectively, the techniques presented here enable a broad spectrum of experiments to study exocrine secretion.

N-wasp is required for structural integrity of the blood-testis barrier

During spermatogenesis, the blood-testis barrier (BTB) segregates the adluminal (apical) and basal compartments in the seminiferous epithelium, thereby creating a privileged adluminal environment that allows post-meiotic spermatid development to proceed without interference of the host immune system. A key feature of the BTB is its continuous remodeling within the Sertoli cells, the major somatic component of the seminiferous epithelium. This remodeling is necessary to allow the transport of germ cells towards the seminiferous tubule interior, while maintaining intact barrier properties. Here we demonstrate that the actin nucleation promoting factor Neuronal Wiskott-Aldrich Syndrome Protein (N-WASP) provides an essential function necessary for BTB restructuring, and for maintaining spermatogenesis. Our data suggests that the N-WASP-Arp2/3 actin polymerization machinery generates branched-actin arrays at an advanced stage of BTB remodeling. These arrays are proposed to mediate the restructuring process through endocytic recycling of BTB components. Disruption of N-WASP in Sertoli cells results in major structural abnormalities to the BTB, including mis-localization of critical junctional and cytoskeletal elements, and leads to disruption of barrier function. These impairments result in a complete arrest of spermatogenesis, underscoring the critical involvement of the somatic compartment of the seminiferous tubules in germ cell maturation.

Mammalian spermatogenesis takes place within a sheltered environment, whereby somatic Sertoli cells protect and guide germ cells as they mature and differentiate. A key structure generated by the protective Sertoli cell epithelium is the blood-testis barrier (BTB), a composite of junctional and cytoskeletal elements, which prevents exposure of post-meiotic spermatids to the immune system. The BTB is a highly dynamic structure, which needs to be dismantled and rapidly rebuilt, in order to allow passage of maturing preleptotene spermatocytes, without compromising their isolation. Here we show that N-WASP, a conserved facilitator of formation of branched actin microfilament arrays, provides a function that is essential for maintenance of an intact BTB. Genetic disruption of N-WASP in mouse Sertoli cells leads to loss of BTB impermeability, resulting in a complete arrest of spermatogenesis at early and post-meiotic stages. Based on the localization patterns of key elements, we propose that branched-actin filaments participate in recycling of BTB materials to ensure the dynamic and efficient maintenance of this structure, one of a series of blood-tissue barriers that preserve privileged organ environments.

Sequential activation of ETS proteins provides a sustained transcriptional response to EGFR signaling

How signal transduction, which is dynamic and fluctuating by nature, is converted into a stable trancriptional response, is an unanswered question in developmental biology. Two ETS-domain transcription factors encoded by the pointed (pnt) locus, PntP1 and PntP2, are universal downstream mediators of EGFR-based signaling in Drosophila. Full disruption of pnt function in developing eye imaginal discs reveals a photoreceptor recruitment phenotype, in which only the R8 photoreceptor cell type is specified within ommatidia. Specific disruption of either pntP1 or pntP2 resulted in the same R8-only phenotype, demonstrating that both Pnt isoforms are essential for photoreceptor recruitment. We show that the two Pnt protein forms are activated in a sequential manner within the EGFR signaling pathway: MAPK phosphorylates and activates PntP2, which in turn induces pntP1 transcription. Once expressed, PntP1 is constitutively active and sufficient to induce target genes essential for photoreceptor development. Pulse-chase experiments indicate that PntP1 is stable for several hours in the eye disc. Sequential ETS-protein recruitment therefore allows sustained induction of target genes, beyond the transient activation of EGFR.

Targeting secretion to the apical surface by mDia1-built actin tracks

The apical surface of secretory tubular epithelia is a dynamic cellular domain where massive membrane turnover takes place during exocytosis and its subsequent compensatory endocytosis. This extensive membrane flow poses a difficulty in targeting secretory vesicles efficiently to a narrow apical domain. We have studied how actin filaments mediate the secretory process in the murine exocrine pancreas, which produces and secretes digestive enzymes that are deposited into the intestine. We show that cargo-filled secretory vesicles move over bundles of linear actin cables from their storage areas to the apical membrane of pancreatic acinar cells. mDia1, a linear actin nucleator of the Formin family, was identified as the generator of these structures. The active form of mDia1 localizes to the apical surface, and the microfilament bundles it forms emanate from the apical surface and extend into the cytoplasm, generating polarized secretion tracks. These bundles ensure orderly progression of exocytosis, since the apical targeting of pancreatic vesicles is compromised in their absence, and vesicles fuse with each other to generate compound, membrane-associated secretory structures.

Apical targeting of the formin Diaphanous in Drosophila tubular epithelia

Apical secretion from epithelial tubes of the Drosophila embryo is mediated by apical F-actin cables generated by the formin-family protein Diaphanous (Dia). Apical localization and activity of Dia are at the core of restricting F-actin formation to the correct membrane domain. Here we identify the mechanisms that target Dia to the apical surface. PI(4,5)P2 levels at the apical membrane regulate Dia localization in both the MDCK cyst model and in Drosophila tubular epithelia. An N-terminal basic domain of Dia is crucial for apical localization, implying direct binding to PI(4,5)P2. Dia apical targeting also depends on binding to Rho1, which is critical for activation-induced conformational change, as well as physically anchoring Dia to the apical membrane. We demonstrate that binding to Rho1 facilitates interaction with PI(4,5)P2 at the plane of the membrane. Together these cues ensure efficient and distinct restriction of Dia to the apical membrane. DOI:http://dx.doi.org/10.7554/eLife.00666.001.

Creating gradients by morphogen shuttling

Morphogen gradients are used to pattern a field of cells according to variations in the concentration of a signaling molecule. Typically, the morphogen emanates from a confined group of cells. During early embryogenesis, however, the ability to define a restricted source for morphogen production is limited. Thus, various early patterning systems rely on a broadly expressed morphogen that generates an activation gradient within its expression domain. Computational and experimental work has shed light on how a sharp and robust gradient can be established under those situations, leading to a mechanism termed 'morphogen shuttling'. This mechanism relies on an extracellular shuttling molecule that forms an inert, highly diffusible complex with the morphogen. Morphogen release from the complex following cleavage of the shuttling molecule by an extracellular protease leads to the accumulation of free ligand at the center of its expression domain and a graded activation of the developmental pathway that decreases significantly even within the morphogen-expression domain.

Orienting the direction of EGFR activation

Morphogens are typically distributed symmetrically from their source of production. In this issue of Developmental Cell, Peng et al. (2012) demonstrate that a bias in the directionality of protrusions emanating from cells secreting the EGFR ligand Spitz leads to asymmetric activation of the pathway.

Bidirectional Notch activation represses fusion competence in swarming adult Drosophila myoblasts

A major aspect of indirect flight muscle formation during adult Drosophila myogenesis involves transition of a semi-differentiated and proliferating pool of myoblasts to a mature myoblast population, capable of fusing with nascent myotubes and generating mature muscle fibers. Here we examine the molecular genetic programs underlying these two phases of myoblast differentiation. We show that the cell adhesion proteins Dumbfounded (Duf) and Sticks and stones (Sns), together with their paralogs Roughest (Rst) and Hibris (Hbs), respectively, are required for adhesion of migrating myoblasts to myotubes and initiation of myoblast-myotube fusion. As myoblasts approach their myotube targets, they are maintained in a semi-differentiated state by continuous Notch activation, where each myoblast provides the ligand Delta to its neighbors. This unique form of bidirectional Notch activation is achieved by finely tuning the levels of the ligand and receptor. Activation of Notch signaling in myoblasts represses expression of key fusion elements such as Sns. Only upon reaching the vicinity of the myotubes does Notch signaling decay, leading to terminal differentiation of the myoblasts. The ensuing induction of proteins required for fusion enables myoblasts to fuse with the myotubes and give rise to subsequent muscle fiber growth.

Self-organized shuttling: generating sharp dorsoventral polarity in the early Drosophila embryo

Morphogen gradients pattern tissues and organs during development. When morphogen production is spatially restricted, diffusion and degradation are sufficient to generate sharp concentration gradients. It is less clear how sharp gradients can arise within the source of a broadly expressed morphogen. A recent solution relies on localized production of an inhibitor outside the domain of morphogen production, which effectively redistributes (shuttles) and concentrates the morphogen within its expression domain. Here, we study how a sharp gradient is established without a localized inhibitor, focusing on early dorsoventral patterning of the Drosophila embryo, where an active ligand and its inhibitor are concomitantly generated in a broad ventral domain. Using theory and experiments, we show that a sharp Toll activation gradient is produced through "self-organized shuttling," which dynamically relocalizes inhibitor production to lateral regions, followed by inhibitor-dependent ventral shuttling of the activating ligand Spätzle. Shuttling may represent a general paradigm for patterning early embryos.

Regulation of developmental intercellular signalling by intracellular trafficking

Universal trafficking components within the cell can be recruited to coordinate and regulate the developmental signalling cascades. We will present ways in which the intracellular trafficking machinery is used to affect and modulate the outcome of signal transduction in developmental contexts, thus regulating multicellular development. Each of the signalling components must reach its proper intracellular destination, in a form that is properly folded and modified. In many instances, the ability to bring components together or segregate them into distinct compartments within the cell actually provides the switch mechanism to turn developmental signalling pathways on or off. The review will begin with a focus on the signal-sending cells, and the ways in which ligand trafficking can impinge on the signalling outcome, via processing, endocytosis and recycling. We will then turn to the signal-receiving cell, and discuss mechanisms by which endocytosis can affect the spatial features of the signal, and the compartmentalization of components downstream to the receptor.

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.

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.

Born to run: creating the muscle fiber

From the muscles that control the blink of your eye to those that allow you to walk, the basic architecture of muscle is the same: muscles consist of bundles of the unit muscle cell, the muscle fiber. The unique morphology of the individual muscle fiber is dictated by the functional demands necessary to generate and withstand the forces of contraction, which in turn leads to movement. Contractile muscle fibers are elongated, syncytial cells, which interact with both the nervous and skeletal systems to govern body motion. In this review, we focus on three key cell-cell and cell-matrix contact processes, that are necessary to create this exquisitely specialized cell: cell fusion, cell elongation, and establishment of a myotendinous junction. We address these processes by highlighting recent findings from the Drosophila model system.

Generation of distinct signaling modes via diversification of the Egfr ligand-processing cassette

Egfr ligand processing in Drosophila involves trafficking of the ligand precursor by the chaperone Star from the endoplasmic reticulum (ER) to a secretory compartment, where the precursor is cleaved by the intramembrane protease Rhomboid. Some of the Drosophila Rhomboids also reside in the ER, where they attenuate signaling by premature cleavage of Star. The genome of the flour beetle Tribolium castaneum contains a single gene for each of the ligand-processing components, providing an opportunity to assess the regulation and impact of a simplified ligand-processing cassette. We find that the central features of ligand retention, trafficking by the chaperone and cleavage by Rhomboid have been conserved. The single Rhomboid is localized to both ER and secretory compartments. However, we show that Tribolium Star is refractive to Rhomboid cleavage. Consequently, this ligand-processing system effectively mediates long-range Egfr activation in the Tribolium embryonic ventral ectoderm, despite ER localization of Rhomboid. Diversification of the Egfr signaling pathway appears to have coupled gene duplication events with modulation of the biochemical properties and subcellular localization patterns of Rhomboid proteases and their substrates.

Polarized secretion of Drosophila EGFR ligand from photoreceptor neurons is controlled by ER localization of the ligand-processing machinery

The release of signaling molecules from neurons must be regulated, to accommodate their highly polarized structure. In the developing Drosophila visual system, photoreceptor neurons secrete the epidermal growth factor receptor ligand Spitz (Spi) from their cell bodies, as well as from their axonal termini. Here we show that subcellular localization of Rhomboid proteases, which process Spi, determines the site of Spi release from neurons. Endoplasmic reticulum (ER) localization of Rhomboid 3 is essential for its ability to promote Spi secretion from axons, but not from cell bodies. We demonstrate that the ER extends throughout photoreceptor axons, and show that this feature facilitates the trafficking of the Spi precursor, the ligand chaperone Star, and Rhomboid 3 to axonal termini. Following this trafficking step, secretion from the axons is regulated in a manner similar to secretion from cell bodies. These findings uncover a role for the ER in trafficking proteins from the neuronal cell body to axon terminus.

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

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

Delta traffic takes a sh-Arp turn

In the Notch pathway, the transmembrane ligand Delta is internalized and then re-established on the surface of signal-sending cells to allow the productive binding and activation of the Notch receptor on neighbouring cells. Arp2/3-dependent actin polymerization directs Delta trafficking through this circuit.

Drosophila EGFR signalling is modulated by differential compartmentalization of Rhomboid intramembrane proteases

We explore the role of differential compartmentalization of Rhomboid (Rho) proteases that process the Drosophila EGF receptor ligands, in modulating the amount of secreted ligand and consequently the level of EGF receptor (EGFR) activation. The mSpitz ligand precursor is retained in the ER, and is trafficked by the chaperone Star to a late compartment of the secretory pathway, where Rho-1 resides. This work demonstrates that two other Rho proteins, Rho-2 and Rho-3, which are expressed in the germ line and in the developing eye, respectively, cleave the Spitz precursor and Star already in the ER, in addition to their activity in the late compartment. This property attenuates EGFR activation, primarily by compromising the amount of chaperone that can productively traffic the ligand precursor to the late compartment, where cleavage and subsequent secretion take place. These observations identify changes in intracellular compartment localization of Rho proteins as a basis for signal attenuation, in tissues where EGFR activation must be highly restricted in space and time.

Drosophila follicle cells are patterned by multiple levels of Notch signaling and antagonism between the Notch and JAK/STAT pathways

The specification of polar, main-body and stalk follicle cells in the germarium of the Drosophila ovary plays a key role in the formation of the egg chamber and polarisation of its anterior-posterior axis. High levels of Notch pathway activation, resulting from a germline Delta ligand signal, induce polar cells. Here we show that low Notch activation levels, originating from Delta expressed in the polar follicle cells, are required for stalk formation. The metalloprotease Kuzbanian-like, which cleaves and inactivates Delta, reduces the level of Delta signaling between follicle cells, thereby limiting the size of the stalk. We find that Notch activation is required in a continuous fashion to maintain the polar and stalk cell fates. We further demonstrate that mutual antagonism between the Notch and JAK/STAT signaling pathways provides a crucial facet of follicle cell patterning. Notch signaling in polar and main-body follicle cells inhibits JAK/STAT signaling by preventing STAT nuclear translocation, thereby restricting the influence of this pathway to stalk cells. Conversely, signaling by JAK/STAT reduces Notch signaling in the stalk. Thus, variations in the levels of Notch pathway activation, coupled with a continuous balance between the Notch and JAK/STAT pathways, specify the identity of the different follicle cell types and help establish the polarity of the egg chamber.

WIP/WASp-based actin-polymerization machinery is essential for myoblast fusion in Drosophila

Formation of syncytial muscle fibers involves repeated rounds of cell fusion between growing myotubes and neighboring myoblasts. We have established that Wsp, the Drosophila homolog of the WASp family of microfilament nucleation-promoting factors, is an essential facilitator of myoblast fusion in Drosophila embryos. D-WIP, a homolog of the conserved Verprolin/WASp Interacting Protein family of WASp-binding proteins, performs a key mediating role in this context. D-WIP, which is expressed specifically in myoblasts, associates with both the WASp-Arp2/3 system and with the myoblast adhesion molecules Dumbfounded and Sticks and Stones, thereby recruiting the actin-polymerization machinery to sites of myoblast attachment and fusion. Our analysis demonstrates that this recruitment is normally required late in the fusion process, for enlargement of nascent fusion pores and breakdown of the apposed cell membranes. These observations identify cellular and developmental roles for the WASp-Arp2/3 pathway, and provide a link between force-generating actin polymerization and cell fusion.

Microtubule-dependent organization of subcortical microfilaments in the earlyDrosophila embryo

Dynamic alterations in the spatial organization of cytoskeletal elements constitute a prominent morphological feature of the early, syncytial stages of embryogenesis in Drosophila. Here, we describe and characterize the dynamic behavior of cytoplasmic, subcortical microfilaments, which form a series of nucleus-associated structures, at different phases of the simultaneous nuclear division cycles characteristic of early Drosophila embryos. Remodeling of the cytoplasmic microfilament arrays takes place in parallel to the established cyclic reorganization of cortical microfilament structures. We provide evidence that the cortical and subcortical microfilament populations organize independently of each other, and in response to distinct instructive cues. Specifically, formation of subcortical microfilament structures appears to rely on, and spatially mirror, the organization of polarized microtubule arrays, while cortical microfilament restructuring constitutes a centrosome-dependent process. Genetic analysis identifies a requirement for SCAR, a key mediator of Arp2/3-based microfilament dynamics, in organization of subcortical microfilament structures.

Actin organization in the early Drosophila embryo

Organization of the cortical cytoplasm during the syncytial blastoderm stages of early Drosophila embryogenesis relies on cyclic transitions between transient microfilament structures. Microtubule-organizing centres (MTOCs) appear to provide the instructive cues governing this dynamic, cell-cycle-dependent process. Using a genetic approach, we have identified key roles for two molecular pathways in mediating these events. The conserved Arp2/3 microfilament nucleation machinery, likely acting in response to the activating element SCAR, plays an essential role in establishment of a cortical F-actin array, and contributes to specific aspects of cyclic microfilament restructuring. Defective cortical microfilament organization is the primary phenotypic feature of embryos derived from mothers bearing mutations in the sponge locus. Several lines of investigation suggest that the primary defect in sponge lies in a faulty cortical microfilament response, downstream of the centrosomal signal. We have determined that sponge encodes a Drosophila homologue of the evolutionarily-conserved CDM (DOCK180) protein family.

Modular tubes: common principles of renal development

The nephrons of the vertebrate kidney originate from mesenchymal tissue that is recruited and incorporated into a branching epithelium. Key features of this unusual manner of specifying functional units within a tubular organ have now been found to be similarly employed during development of the insect renal system.

Conserved interactions with cytoskeletal but not signaling elements are an essential aspect of Drosophila WASp function

Wiskott-Aldrich Syndrome proteins (WASp) serve as important regulators of cytoskeletal organization and function. These modular proteins, which are well-conserved among eukaryotic species, act to promote actin filament assembly in response to cues from various signal transduction pathways. Genetic analysis has revealed a requirement for the single Drosophila homolog, Wasp (Wsp), in cell-fate decisions governing specific neuronal lineages. We have used this unique developmental context to assess the contributions of established signaling and cytoskeletal partners of WASp. We present biochemical and genetic evidence that, as expected, Drosophila Wsp performs its developmental role via the Arp2/3 complex, indicating conservation of the cytoskeletal aspect of Wsp function in vivo. In contrast, we find that association with the key signaling molecules CDC42 and PIP2 is not an essential requirement, implying that activation of Wsp function in vivo depends on additional or alternative signaling pathways.

SCAR is a primary regulator of Arp2/3-dependent morphological events in Drosophila

The Arp2/3 complex and its activators, Scar/WAVE and Wiskott-Aldrich Syndrome protein (WASp), promote actin polymerization in vitro and have been proposed to influence cell shape and motility in vivo. We demonstrate that the Drosophila Scar homologue, SCAR, localizes to actin-rich structures and is required for normal cell morphology in multiple cell types throughout development. In particular, SCAR function is essential for cytoplasmic organization in the blastoderm, axon development in the central nervous system, egg chamber structure during oogenesis, and adult eye morphology. Highly similar developmental requirements are found for subunits of the Arp2/3 complex. In the blastoderm, SCAR and Arp2/3 mutations result in a reduction in the amount of cortical filamentous actin and the disruption of dynamically regulated actin structures. Remarkably, the single Drosophila WASp homologue, Wasp, is largely dispensable for these numerous Arp2/3-dependent functions, whereas SCAR does not contribute to cell fate decisions in which Wasp and Arp2/3 play an essential role. These results identify SCAR as a major component of Arp2/3-dependent cell morphology during Drosophila development and demonstrate that the Arp2/3 complex can govern distinct cell biological events in response to SCAR and Wasp regulation.

Conserved domains of the Nullo protein required for cell-surface localization and formation of adherens junctions

During cellularization, the Drosophila melanogaster embryo undergoes a transition from syncytial to cellular blastoderm with the de novo generation of a polarized epithelial sheet in the cortex of the embryo. This process couples cytokinesis with the establishment of apical, basal, and lateral membrane domains that are separated by two spatially distinct adherens-type junctions. In nullo mutant embryos, basal junctions fail to form at the onset of cellularization, leading to the failure of cleavage furrow invagination and the generation of multinucleate cells. Nullo is a novel protein that appears to stabilize the initial accumulation of cadherins and catenins as they form a mature basal junction. In this article we characterize a nullo homologue from D. virilis and identify conserved domains of Nullo that are required for basal junction formation. We also demonstrate that Nullo is a myristoylprotein and that the myristate group acts in conjunction with a cluster of basic amino acids to target Nullo to the plasma membrane. The membrane association of Nullo is required in vivo for its role in basal junction formation and for its ability to block apical junction formation when ectopically expressed during late cellularization.

Wasp, the Drosophila Wiskott-Aldrich syndrome gene homologue, is required for cell fate decisions mediated by Notch signaling

Wiskott-Aldrich syndrome proteins, encoded by the Wiskott-Aldrich syndrome gene family, bridge signal transduction pathways and the microfilament-based cytoskeleton. Mutations in the Drosophila homologue, Wasp (Wsp), reveal an essential requirement for this gene in implementation of cell fate decisions during adult and embryonic sensory organ development. Phenotypic analysis of Wsp mutant animals demonstrates a bias towards neuronal differentiation, at the expense of other cell types, resulting from improper execution of the program of asymmetric cell divisions which underlie sensory organ development. Generation of two similar daughter cells after division of the sensory organ precursor cell constitutes a prominent defect in the Wsp sensory organ lineage. The asymmetric segregation of key elements such as Numb is unaffected during this division, despite the misassignment of cell fates. The requirement for Wsp extends to additional cell fate decisions in lineages of the embryonic central nervous system and mesoderm. The nature of the Wsp mutant phenotypes, coupled with genetic interaction studies, identifies an essential role for Wsp in lineage decisions mediated by the Notch signaling pathway.

Threonine phosphorylation diverts internalized epidermal growth factor receptors from a degradative pathway to the recycling endosome

Transregulation of the epidermal growth factor receptor (EGFR) by protein kinase C (PKC) serves as a model for heterologous desensitization of receptor tyrosine kinases, but the underlying mechanism remained unknown. By using c-Cbl-induced ubiquitination of EGFR as a marker for transfer from early to late endosomes, we provide evidence that PKC can inhibit this process. In parallel, receptor down-regulation and degradation are significantly reduced. The inhibitory effects of PKC are mediated by a single threonine residue (threonine 654) of EGFR, which serves as a major PKC phosphorylation site. Biochemical and morphological analyses indicate that threonine-phosphorylated EGFR molecules undergo normal internalization, but instead of sorting to lysosomal degradation, they recycle back to the cell surface. In conclusion, by sorting EGFR to the recycling endosome, heterologous desensitization restrains ligand-induced down-regulation of EGFR.

Mutations in centrosomin reveal requirements for centrosomal function during early Drosophila embryogenesis

BACKGROUND

Although centrosomes serve as the primary organizing centers for the microtubule-based cytoskeleton in animal cells, various studies question the requirements for these organelles during the formation of microtubule arrays and execution of microtubule-dependent processes. Using a genetic approach to interfere with centrosomal function, we present an assessment of this issue, in the context of early embryogenesis of the fruit fly Drosophila melanogaster.

RESULTS

We identified mutant alleles of the centrosomin (cnn) locus, which encodes a core component of centrosomes in Drosophila. The cnn mutant flies were viable but sterile. The normal course of early embryonic development was arrested in all progeny of cnn mutant females. Our analysis identified a failure to form functional centrosomes and spindle poles as the primary mutant phenotype of cnn embryos. Various aspects of early development that are dependent on cytoskeletal control were disrupted in cnn mutant embryos. In particular, structural rearrangements of cortical microfilaments were strongly dependent on proper centrosomal function.

CONCLUSIONS

Centrosomin is an essential core component of early embryonic centrosomes in Drosophila. Microtubule-dependent events of early embryogenesis display differential requirements for centrosomal function.

bottleneck acts as a regulator of the microfilament network governing cellularization of the Drosophila embryo

A dynamic network of cortical microfilaments is associated with the cleavage furrow membranes during cellularization of the Drosophila embryo. A specific set of structural rearrangements in this network is required for orchestration and execution of its mechanistic roles. We describe the characterization of the gene bottleneck (bnk), mutations in which disturb the proper sequence of rearrangements of the microfilament network, leading to a variety of morphological defects during cellularization. bnk, whose expression is restricted to the blastoderm stages of Drosophila embryogenesis, encodes a novel, exceptionally basic protein that specifically colocalizes with the microfilament network. The expression pattern and mutant phenotype of bnk suggest a direct role for this element in regulation of the dynamic restructuring of the actin-based cytoskeleton of cellularizing Drosophila embryos.

Interallelic complementation among DER/flb alleles: implications for the mechanism of signal transduction by receptor-tyrosine kinases

The large number of available embryonic lethal alleles in the Drosophila EGF receptor homolog (DER)/faint little ball locus allowed us to test the possibility of positive or negative interactions among different DER alleles. These interactions were monitored by examining the embryonic cuticular phenotypes of different heteroallelic combinations. Several positive interactions were identified, while negative interactions were restricted to a single allele. This is the first example of positive interactions within the same cell type among alleles of a receptor tyrosine kinase gene. The basis for these interactions is likely to arise from the mechanism of signal transduction by receptor tyrosine kinases, which involves receptor aggregation. A combination of two different DER mutant proteins defective in temporally distinct stages of the signal transduction process, may thus form a functional heterodimer. The mutation sites in four alleles showing positive interactions were localized. They identify regions within the protein which are likely to be important for these temporally distinct signal transduction processes.