Accumulation of the Drosophila Torso-like protein at the blastoderm plasma membrane suggests that it translocates from the eggshell

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

Vitelline Membrane Protein 26 Mutagenesis, Using CRISPR/Cas9, Results in Egg Collapse in Plutella xylostella

Vitelline membrane proteins (VMPs) are the main proteins that form the inner shell (vitelline membrane layer) of insect eggs and are an integral part of egg formation and embryo development. Here, we characterized the molecular structure and expression patterns of the VMP26 gene and analyzed its reproductive functions in diamondback moth, Plutella xylostella (L.), a worldwide migratory pest of cruciferous plants. The PxVMP26 gene was shown to be a single exon gene that contained an open reading frame of 852 base pairs (bp) encoding 283 amino acids. Both qPCR and western blot analyses showed that PxVMP26 was specifically expressed in female adults and was significantly highly expressed in the ovary. Further anatomical analysis indicated that the expression level of PxVMP26 in the ovarian tube with an incomplete yolk was significantly higher than that in the ovarian tube with a complete yolk. CRISPR/Cas9-induced PxVMP26 knockout successfully created two homozygous strains with 8- and 46-bp frameshift mutations. The expression deficiency of the PxVMP26 protein was detected in the mutant strains using immunofluorescence and western blot. No significant difference was found in the number of eggs laid within three days between wild and mutant individuals, but there was a lower egg hatchability. The loss of the PxVMP26 gene changed the mean egg size, damaged the structure of the vitelline membrane, and increased the proportion of abnormal eggs due to water loss, resulting in egg collapse. This first analysis of the roles of the VMP gene in the oocyte formation and embryonic development of P. xylostella, using CRISPR/Cas9 technology, provides a basis for screening new genetic control targets of P. xylostella.

Setting up for gastrulation: D. melanogaster

Drosophila melanogaster embryos develop initially as a syncytium of totipotent nuclei and subsequently, once cellularized, undergo morphogenetic movements associated with gastrulation to generate the three somatic germ layers of the embryo: mesoderm, ectoderm, and endoderm. In this chapter, we focus on the first phase of gastrulation in Drosophila involving patterning of early embryos when cells differentiate their gene expression programs. This patterning process requires coordination of multiple developmental processes including genome reprogramming at the maternal-to-zygotic transition, combinatorial action of transcription factors to support distinct gene expression, and dynamic feedback between this genetic patterning by transcription factors and changes in cell morphology. We discuss the gene regulatory programs acting during patterning to specify the three germ layers, which involve the regulation of spatiotemporal gene expression coupled to physical tissue morphogenesis.

Isolation of secreted proteins from Drosophila ovaries and embryos through in vivo BirA-mediated biotinylation

The extraordinarily strong non-covalent interaction between biotin and avidin (kD = 10-14-10-16) has permitted this interaction to be used in a wide variety of experimental contexts. The Biotin Acceptor Peptide (BAP), a 15 amino acid motif that can be biotinylated by the E. coli BirA protein, has been fused to proteins-of-interest, making them substrates for in vivo biotinylation. Here we report on the construction and characterization of a modified BirA bearing signals for secretion and endoplasmic reticulum (ER) retention, for use in experimental contexts requiring biotinylation of secreted proteins. When expressed in the Drosophila female germline or ovarian follicle cells under Gal4-mediated transcriptional control, the modified BirA protein could be detected and shown to be enzymatically active in ovaries and progeny embryos. Surprisingly, however, it was not efficiently retained in the ER, and instead appeared to be secreted. To determine whether this secreted protein, now designated secBirA, could biotinylate secreted proteins, we generated BAP-tagged versions of two secreted Drosophila proteins, Torsolike (Tsl) and Gastrulation Defective (GD), which are normally expressed maternally and participate in embryonic pattern formation. Both Tsl-BAP and GD-BAP were shown to exhibit normal patterning activity. Co-expression of Tsl-BAP together with secBirA in ovarian follicle cells resulted in its biotinylation, which permitted its isolation from both ovaries and progeny embryos using Avidin-coupled affinity matrix. In contrast, co-expression with secBirA in the female germline did not result in detectable biotinylation of GD-BAP, possibly because the C-terminal location of the BAP tag made it inaccessible to BirA in vivo. Our results indicate that secBirA directs biotinylation of proteins bound for secretion in vivo, providing access to powerful experimental approaches for secreted proteins-of-interest. However, efficient biotinylation of target proteins may vary depending upon the location of the BAP tag or other structural features of the protein.

The torso-like gene functions to maintain the structure of the vitelline membrane in Nasonia vitripennis, implying its co-option into Drosophila axis formation

Axis specification is a fundamental developmental process. Despite this, the mechanisms by which it is controlled across insect taxa are strikingly different. An excellent example of this is terminal patterning, which in Diptera such as Drosophila melanogaster occurs via the localized activation of the receptor tyrosine kinase Torso. In Hymenoptera, however, the same process appears to be achieved via localized mRNA. How these mechanisms evolved and what they evolved from remains largely unexplored. Here, we show that torso-like, known for its role in Drosophila terminal patterning, is instead required for the integrity of the vitelline membrane in the hymenopteran wasp Nasonia vitripennis. We find that other genes known to be involved in Drosophila terminal patterning, such as torso and Ptth, also do not function in Nasonia embryonic development. These findings extended to orthologues of Drosophila vitelline membrane proteins known to play a role in localizing Torso-like in Drosophila; in Nasonia these are instead required for dorso–ventral patterning, gastrulation and potentially terminal patterning. Our data underscore the importance of the vitelline membrane in insect development, and implies phenotypes caused by knockdown of torso-like must be interpreted in light of its function in the vitelline membrane. In addition, our data imply that the signalling components of the Drosophila terminal patterning systems were co-opted from roles in regulating moulting, and co-option into terminal patterning involved the evolution of a novel interaction with the vitelline membrane protein Torso-like.

This article has an associated First Person interview with the first author of the paper.

Summary: In the parasitic wasp Nasonia, Tsl, a key component of the process that defines the termini of the embryo of Drosophila, has a function in the structure of the vitelline membrane.

Control of growth factor signalling by MACPF proteins

Members of the membrane attack complex/perforin-like (MACPF) protein superfamily have long captured interest because of their unique ability to assemble into large oligomeric pores on the surfaces of cells. The best characterised of these act in vertebrate immunity where they function to deliver pro-apoptotic factors or induce the cytolysis and death of targeted cells. Less appreciated, however, is that rather than causing cell death, MACPF proteins have also evolved to control cellular signalling pathways and influence developmental programmes such as pattern formation and neurogenesis. Torso-like (Tsl) from the fruit fly Drosophila, for example, functions to localise the activity of a growth factor for patterning its embryonic termini. It remains unclear whether these developmental proteins employ an attenuated form of the classical MACPF lytic pore, or if they have evolved to function via alternative mechanisms of action. In this minireview, we examine the evidence that links pore-forming MACPF proteins to the control of growth factor and cytokine signalling. We will then attempt to reconcile how the MACPF domain may have been repurposed during evolution for developmental events rather than cell killing.

Evolution of the Torso activation cassette, a pathway required for terminal patterning and moulting

Embryonic terminal patterning and moulting are critical developmental processes in insects. In Drosophila and Tribolium both of these processes are regulated by the Torso-activation cassette (TAC). The TAC consists of a common receptor, Torso, ligands Trunk and prothoracicotropic hormone (PTTH), and the spatially restricted protein Torso-like, with combinations of these elements acting mechanistically to activate the receptor in different developmental contexts. In order to trace the evolutionary history of the TAC we determined the presence or absence of TAC components in the genomes of arthropods. Our analyses reveal that Torso, Trunk and PTTH are evolutionarily labile components of the TAC with multiple individual or combined losses occurring in the arthropod lineages leading to and within the insects. These losses are often correlated, with both ligands and receptor missing from the genome of the same species. We determine that the PTTH gene evolved in the common ancestor of Hemiptera and Holometabola, and is missing from the genomes of a number of species with experimentally demonstrated PTTH activity, implying another molecule may be involved in ecdysis in these species. In contrast, the torso-like gene is a common component of pancrustacean genomes.

The trigger (and the restriction) of Torso RTK activation

The Torso pathway is an ideal model of receptor tyrosine kinase systems, in particular when addressing questions such as how receptor activity is turned on and, equally important, how it is restricted, how different outcomes can be generated from a single signal, and the extent to which gene regulation by signalling pathways relies on the relief of transcriptional repression. In this regard, we considered it pertinent to single out the fundamental notions learned from the Torso pathway beyond the specificities of this system (Furriols and Casanova 2003 EMBO J. 22, 1947-1952. ( doi:10.1093/emboj/cdg224 )). Since then, the Torso system has gained relevance and its implications beyond its original involvement in morphogenesis and into many disciplines such as oncogenesis, hormone control and neurobiology are now acknowledged. Thus, we believe that it is timely to highlight additional notions supported by new findings and to draw attention to future prospects. Given the late development of research in the field, we wish to devote this review to the events leading to the activation of the Torso receptor, the main focus of our most recent work.

Toxicity and sublethal effects of fluralaner on Spodoptera litura Fabricius (Lepidoptera: Noctuidae)

The increasing occurrence of resistance to chemical insecticides in insect pest populations is a serious threat to the integrity of current pest management strategies, and exploring new alternative chemistries is one important way to overcome this obstacle. Fluralaner, as a novel isoxazoline insecticide, has broad spectrum activity against a variety of insect pests, but little data is available about its effect on Lepidopterans. The effects of fluralaner on Spodoptera litura Fabricius, a widespread and polyphagous pest, were evaluated in the present study. Our results showed younger larvae were more susceptible to fluralaner treatment, but feeding and topical applications were similarly effective in 3rd instar larvae. Synergism assays indicated that piperonyl butoxide (PBO) could increase the toxicity of fluralaner to S. litura to a certain degree and P450 may be involved in the detoxification of fluralaner in vivo. Sublethal developmental effects included reduced larval body weight, decreased pupation and emergence, and notched wings in adults, accompanied by changes in the transcript levels of chitinase 5 (CHT5) and juvenile hormone acid methyltransferase (Jhamt), genes vital for insect development. Above results manifested that fluralaner is highly toxic to S. litura larvae via either topical or oral application and provide an indication of how this insecticide is metabolized in vivo. Further, our results provided a foundation for further development of fluralaner as a new tool in insect pest management.

Holes in the Plasma Membrane Mimic Torso-Like Perforin in Torso Tyrosine Kinase Receptor Activation in the Drosophila Embryo

Receptor tyrosine kinase (RTK) pathways play central roles in development, and, when abnormally activated, they can lead to pathological conditions, including oncogenesis. Thus, RTK activation, mediated by ligand binding, is under tight control, a critical step being the conversion of an inactive precursor into the active form of the ligand. A variety of mechanisms have been shown to be involved in this conversion; however, little attention has been paid to how mechanical phenomena may impinge on this process. Here we address this issue by studying Torso, an RTK activated at both poles of the Drosophila embryo at the blastoderm stage. Torso activation is induced by a cleaved form of Trunk, a growth factor-like protein, but it also requires the accumulation of the Torso-like (Tsl) protein at both ends of the blastoderm. Tsl is the only known protein in Drosophila bearing a membrane attack complex/perforin (MACPF) domain-a motif present in proteins involved in pore formation at cell membranes. However, while different hypotheses have been put forward to account for the function of Tsl in Torso receptor activation, little is known about its molecular role and whether it indeed contributes to membrane pore formation. Here, we show that mechanically induced holes in the Drosophila embryo can substitute for Tsl function. These results suggest that Tsl is required for an exchange between the interior of the Drosophila embryo and its surrounding milieu and that mechanically induced cell injuries may contribute to abnormal RTK activation.

Repurposing a pore: highly conserved perforin-like proteins with alternative mechanisms

Pore-forming proteins play critical roles in pathogenic attack and immunological defence. The membrane attack complex/perforin (MACPF) group of homologues represents, with cholesterol-dependent cytolysins, the largest family of such proteins. In this review, we begin by describing briefly the structure of MACPF proteins, outlining their common mechanism of pore formation. We subsequently discuss some examples of MACPF proteins likely implicated in pore formation or other membrane-remodelling processes. Finally, we focus on astrotactin and bone morphogenetic protein and retinoic acid-induced neural-specific proteins, highly conserved MACPF family members involved in developmental processes, which have not been well studied to date or observed to form a pore-and which data suggest may act by alternative mechanisms.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

Maternal Torso-Like Coordinates Tissue Folding During Drosophila Gastrulation

The rapid and orderly folding of epithelial tissue during developmental processes such as gastrulation requires the precise coordination of changes in cell shape. Here, we report that the perforin-like protein Torso-like (Tsl), the key extracellular determinant for Drosophila embryonic terminal patterning, also functions to control epithelial morphogenesis. We find that tsl null mutants display a ventral cuticular hole phenotype that is independent of the loss of terminal structures, and arises as a consequence of mesoderm invagination defects. We show that the holes are caused by uncoordinated constriction of ventral cell apices, resulting in the formation of an incomplete ventral furrow. Consistent with these data, we find that loss of tsl is sensitive to gene dosage of RhoGEF2, a critical mediator of Rho1-dependent ventral cell shape changes during furrow formation, suggesting that Tsl may act in this pathway. In addition, loss of tsl strongly suppressed the effects of ectopic expression of Folded Gastrulation (Fog), a secreted protein that promotes apical constriction. Taken together, our data suggest that Tsl controls Rho1-mediated apical constriction via Fog. Therefore, we propose that Tsl regulates extracellular Fog activity to synchronize cell shape changes and coordinate ventral morphogenesis in Drosophila Identifying the Tsl-mediated event that is common to both terminal patterning and morphogenesis will be valuable for our understanding of the extracellular control of developmental signaling by perforin-like proteins.

Transfer of Dorsoventral and Terminal Information from the Ovary to the Embryo by a Common Group of Eggshell Proteins in Drosophila

The Drosophila eggshell is an extracellular matrix that confers protection to the egg and also plays a role in transferring positional information from the ovary to pattern the embryo. Among the constituents of the Drosophila eggshell, Nasrat, Polehole, and Closca form a group of proteins related by sequence, secreted by the oocyte, and mutually required for their incorporation into the eggshell. Besides their role in eggshell integrity, Nasrat, Polehole, and Closca are also required for embryonic terminal patterning by anchoring or stabilizing Torso-like at the eggshell. Here, we show that they are also required for dorsoventral patterning, thereby unveiling that the dorsoventral and terminal systems, hitherto considered independent, share a common extracellular step. Furthermore, we show that Nasrat, Polehole, and Closca are required for proper Nudel activity, a protease acting both in embryonic dorsoventral patterning and eggshell integrity, thus providing a means to account for the role of Nasrat, Polehole, and Closca. We propose that a Nasrat/Polehole/Closca complex acts as a multifunctional hub to anchor various proteins synthesized at oogenesis, ensuring their spatial and temporal restricted function.

Reconstitution of Torso signaling in cultured cells suggests a role for both Trunk and Torso-like in receptor activation

Formation of the Drosophila embryonic termini is controlled by the localized activation of the receptor tyrosine kinase Torso. Both Torso and Torso's presumed ligand, Trunk, are expressed uniformly in the early embryo. Polar activation of Torso requires Torso-like, which is expressed by follicle cells adjacent to the ends of the developing oocyte. We find that Torso expressed at high levels in cultured Drosophila cells is activated by individual application of Trunk, Torso-like or another known Torso ligand, Prothoracicotropic Hormone. In addition to assays of downstream signaling activity, Torso dimerization was detected using bimolecular fluorescence complementation. Trunk and Torso-like were active when co-transfected with Torso and when presented to Torso-expressing cells in conditioned medium. Trunk and Torso-like were also taken up from conditioned medium specifically by cells expressing Torso. At low levels of Torso, similar to those present in the embryo, Trunk and Torso-like alone were ineffective but acted synergistically to stimulate Torso signaling. Our results suggest that Torso interacts with both Trunk and Torso-like, which cooperate to mediate dimerization and activation of Torso at the ends of the Drosophila embryo.

Transcriptome sequencing reveals maelstrom as a novel target gene of the terminal-system in the red flour beetle Tribolium castaneum

Terminal regions of the Drosophila embryo are patterned by the localized activation of the Torso-RTK pathway, which promotes the down-regulation of Capicua. In the short-germ beetle Tribolium, the function of the terminal system appears to be rather different, as the pathway promotes axis elongation and in addition, is required for patterning the extraembryonic serosa at the anterior. Here we show that Torso signalling induces gene expression by relieving CAPICUA-mediated repression also in Tribolium. Given that the majority of Torso target genes remain to be identified, we established a differential gene-expression screen. A subset of 50 putative terminal target genes was screened for functions in early embryonic patterning. Of those, 13 genes show early terminal expression domains and also phenotypes were related to terminal patterning. Among others, we found the PIWI-interacting RNA factor Maelstrom to be crucial for early embryonic polarization. Tc-mael is required for proper serosal size regulation and head morphogenesis. Moreover, Tc-mael promotes growth-zone formation and axis elongation. Our results suggest that posterior patterning by Torso may be realized through Maelstrom depended activation of posterior wnt-domains.

Maternal AP determinants in the Drosophila oocyte and embryo

An animal embryo cannot initiate its journey of forming a new life on its own. It must rely on maternally provided resources and inputs to kick-start its developmental process. In Drosophila, the initial polarities of the embryo along both the anterior-posterior (AP) and dorsal-ventral (DV) axes are also specified by maternal determinants. Over the past several decades, genetic and molecular studies have identified and characterized such determinants, as well as the zygotic genetic regulatory networks that control patterning in the early embryo. Extensive studies of oogenesis have also led to a detailed knowledge of the cellular and molecular interactions that control the formation of a mature egg. Despite these efforts, oogenesis and embryogenesis have been studied largely as separate problems, except for qualitative aspects with regard to maternal regulation of the asymmetric localization of maternal determinants. Can oogenesis and embryogenesis be viewed from a unified perspective at a quantitative level, and can that improve our understanding of how robust embryonic patterning is achieved? Here, we discuss the basic knowledge of the regulatory mechanisms controlling oogenesis and embryonic patterning along the AP axis. We explore properties of the maternal Bicoid gradient in relation to embryo size in search for a unified framework for robust AP patterning. WIREs Dev Biol 2016, 5:562-581. doi: 10.1002/wdev.235 For further resources related to this article, please visit the WIREs website.

Torso-like mediates extracellular accumulation of Furin-cleaved Trunk to pattern the Drosophila embryo termini

Patterning of the Drosophila embryonic termini is achieved by localized activation of the Torso receptor by the growth factor Trunk. Governing this event is the perforin-like protein Torso-like, which is localized to the extracellular space at the embryo poles and has long been proposed to control localized proteolytic activation of Trunk. However, a protease involved in terminal patterning remains to be identified, and the role of Torso-like remains unknown. Here we find that Trunk is cleaved intracellularly by Furin proteases. We further show that Trunk is secreted, and that levels of extracellular Trunk are greatly reduced in torso-like null mutants. On the basis of these and previous findings, we suggest that Torso-like functions to mediate secretion of Trunk, thus providing the mechanism for spatially restricted activation of Torso. Our data represent an alternative mechanism for the spatial control of receptor signalling, and define a different role for perforin-like proteins in eukaryotes.

Activation of the growth factor Trunk patterns the Drosophila embryonic termini but how this is regulated is unclear. Here, Johnson et al. report that Trunk is cleaved intracellularly by Furin proteases, and its extracellular accumulation is then mediated by the perforin-like protein Torso-like.