The synthesis-diffusion-degradation model explains Bicoid gradient formation in unfertilized eggs.

Precise formation of morphogen gradients is essential to the establishment of reproducible pattern in development. Mechanisms proposed for obtaining the requisite precision range from simple models with few parameters to more complex models involving many regulated quantities. The synthesis-diffusion-degradation (SDD) model is a relatively simple model explaining the formation of the Bicoid gradient in Drosophila melanogaster, in which the steady-state characteristic length of the gradient is determined solely by the rates of diffusion and degradation of the morphogen. In this work, we test the SDD model in unfertilized D. melanogaster eggs, which contain a single female pronucleus and lack the nuclear division cycles and other zygotic regulatory processes seen in fertilized eggs. Using two-photon live imaging as well as a novel method for quantitative imaging based on decorrelation of photoswitching waveforms, we find that the Bicoid gradient is longer and shallower in unfertilized eggs as compared to the gradient at the same time points in fertilized eggs. Using a means of measuring the Bicoid lifetime by conjugation to a photoconvertible fluorophore, we find that the lifetime is correspondingly longer in unfertilized eggs, providing qualitative and quantitative agreement with the predictions of the SDD model.

Precise domain specification in the developing Drosophila embryo.

A simple morphogen gradient based on the protein bicoid is insufficient to explain the precise (i.e., similar in all embryos) setting of anteroposterior gene expression domains in the early Drosophila embryo. We present here an alternative model, based on quantitative data, which accounts for all of our observations. The model also explains the robustness of hunchback boundary setting in unnatural environments such as published recently [Luccheta, Nature 434, 1134 (2005)]. The model is based on the existence of a secondary gradient correlated to bicoid through protein degradation by the same agent.

Two new roles for the Drosophila AP patterning system in early morphogenesis.

Although gastrulation is regarded as the stage during Drosophila development when the AP patterning system first influences morphological processes, transcription is regulated in complex patterns already at cycle 10. How soon this transcriptional complexity produces spatial differences in morphology has been unclear. We report on two new processes that establish visible morphological inhomogeneities before the onset of gastrulation. The first of these is the regulation of syncytial nuclear densities in the anterior end of the egg and represents the first zygotically driven AP asymmetry in the embryo. The second process is the generation of a fine-scale pattern in the actin/myosin array during cellularization. We find three domains of different yolk stalk diameters as well as depths of cellularization along the AP axis. These domains are established under the control of the AP patterning system and require bicoid activity. The anterior-most domain is a region of large yolk stalk diameters and corresponds to the region of decreased nuclear densities observed during syncytial stages. The middle domain shows smaller yolk stalk diameters and more rapid cellularization. Its establishment requires wild-type paired activity and thus indirectly requires bicoid. It occurs in a region of the embryo that ultimately gives rise to the cephalic furrow and may account for the effect of paired on that structure during gastrulation. Our results therefore suggest a link between cytoskeletal organization during cellularization and subsequent morphogenetic processes of gastrulation.

Cell and developmental biology--a shared past, an intertwined future.

Cell and developmental biology are distinct disciplines with clear differences in emphasis and domains of interest, yet they also share a common historic origin and benefit from an increasingly productive exchange of insights and influences. Our goal in this commentary is to examine the common origin of cell and developmental biology, to explore ways in which they currently interact, and to consider the connections and differences that exist between these two fields.

Armadillo nuclear import is regulated by cytoplasmic anchor Axin and nuclear anchor dTCF/Pan.

Drosophila melanogaster Armadillo plays two distinct roles during development. It is a component of adherens junctions, and functions as a transcriptional activator in response to Wingless signaling. In the current model, Wingless signal causes stabilization of cytoplasmic Armadillo allowing it to enter the nucleus where it can activate transcription. However, the mechanism of nuclear import and export remains to be elucidated. In this study, we show that two gain-of-function alleles of Armadillo activate Wingless signaling by different mechanisms. The S10 allele was previously found to localize to the nucleus, where it activates transcription. In contrast, the Delta Arm allele localizes to the plasma membrane, and forces endogenous Arm into the nucleus. Therefore, Delta Arm is dependent on the presence of a functional endogenous allele of arm to activate transcription. We show that Delta Arm may function by titrating Axin protein to the membrane, suggesting that it acts as a cytoplasmic anchor keeping Arm out of the nucleus. In axin mutants, Arm is localized to the nuclei. We find that nuclear retention is dependent on dTCF/Pangolin. This suggests that cellular distribution of Arm is controlled by an anchoring system, where various nuclear and cytoplasmic binding partners determine its localization.

Polarized insertion of new membrane from a cytoplasmic reservoir during cleavage of the Drosophila embryo.

Cellularization of the Drosophila embryo is a specialized form of cytokinesis that results in the formation of a polarized epithelium. The mechanisms of membrane growth during cytokinesis are largely unknown. It is also unclear whether membrane growth and polarization represent distinct processes that occur simultaneously or whether growth of the membrane is involved in the emergence of polarity. Using a combination of surface labeling and particles tracking techniques, we monitored the dynamics of marked membrane regions during cellularization. We find that the major source of membrane is intracellular, rather than in the form of a plasma membrane reservoir. Depolymerization of microtubules inhibits the export of a newly synthesized transmembrane protein from the Golgi apparatus to the plasma membrane and simultaneously blocks membrane growth. Membrane insertion occurs in a defined sequence at specific sites, first apical, then apical-lateral. Diffusion of the membrane appears insufficient to compete with the massive local insertion of new membrane. We thus identify a tightly regulated scheme of polarized membrane insertion during cellularization. We propose that such a mechanism could participate in the progressive emergence of apical-basal polarity.

Regulated expression of nullo is required for the formation of distinct apical and basal adherens junctions in the Drosophila blastoderm.

During cellularization, the Drosophila embryo undergoes a large-scale cytokinetic event that packages thousands of syncytial nuclei into individual cells, resulting in the de novo formation of an epithelial monolayer in the cortex of the embryo. The formation of adherens junctions is one of the many aspects of epithelial polarity that is established during cellularization: at the onset of cellularization, the Drosophila beta-catenin homologue Armadillo (Arm) accumulates at the leading edge of the cleavage furrow, and later to the apicolateral region where the zonula adherens precursors are formed. In this paper, we show that the basal accumulation of Arm colocalizes with DE-cadherin and Dalpha-catenin, and corresponds to a region of tight membrane association, which we refer to as the basal junction. Although the two junctions are similar in components and function, they differ in their response to the novel cellularization protein Nullo. Nullo is present in the basal junction and is required for its formation at the onset of cellularization. In contrast, Nullo is degraded before apical junction formation, and prolonged expression of Nullo blocks the apical clustering of junctional components, leading to morphological defects in the developing embryo. These observations reveal differences in the formation of the apical and basal junctions, and offer insight into the role of Nullo in basal junction formation.

A genetic link between morphogenesis and cell division during formation of the ventral furrow in Drosophila.

Stages in development with rapid transitions between mitosis and morphogenesis may require specific mechanisms to coordinate cell shape change. Here we describe a novel mitotic inhibitor that acts during Drosophila gastrulation to counteract String/Cdc25, specifically in the cells that invaginate to form the mesoderm. We have identified two genes, frühstart and tribbles, that are required for this ventral inhibition. tribbles encodes a kinase-related protein whose RNA, however, is also present outside of the ventral region. Effective inhibition of mitosis in the cells of the ventral furrow depends on the transcription factor Snail that triggers the ventral cell shape changes. When overexpressed in a microinjection assay, Tribbles directly inhibits mitosis. We propose that Frühstart and Tribbles form a link between the morphogenetic movements and mitotic control.

Dynein-mediated cargo transport in vivo. A switch controls travel distance.

Cytoplasmic dynein is a microtubule-based motor with diverse cellular roles. Here, we use mutations in the dynein heavy chain gene to impair the motor's function, and employ biophysical measurements to demonstrate that cytoplasmic dynein is responsible for the minus end motion of bidirectionally moving lipid droplets in early Drosophila embryos. This analysis yields an estimate for the force that a single cytoplasmic dynein exerts in vivo (1.1 pN). It also allows us to quantitate dynein-mediated cargo motion in vivo, providing a framework for investigating how dynein's activity is controlled. We identify three distinct travel states whose general features also characterize plus end motion. These states are preserved in different developmental stages. We had previously provided evidence that for each travel direction, single droplets are moved by multiple motors of the same type (Welte et al. 1998). Droplet travel distances (runs) are much shorter than expected for multiple motors based on in vitro estimates of cytoplasmic dynein processivity. Therefore, we propose the existence of a process that ends runs before the motors fall off the microtubules. We find that this process acts with a constant probability per unit distance, and is typically coupled to a switch in travel direction. A process with similar properties governs plus end motion, and its regulation controls the net direction of transport.

The Drosophila gene brinker reveals a novel mechanism of Dpp target gene regulation.

decapentaplegic (dpp), a Drosophila member of the TGFbeta family of secreted molecules, functions as a long-range morphogen in patterning of the embryo and the adult appendages. Dpp signals via the SMAD proteins Mad and Medea. Here we show that in the absence of brinker (brk), Mad is not required for the activation of Dpp target genes that depend on low levels of Dpp. brk encodes a novel protein with features of a transcriptional repressor. brk itself is negatively regulated by Dpp. Dpp signaling might relieve brk's repression of low-level target genes either by transcriptional repression of brk or by antagonizing a repressor function of brk at the target gene promoters.

Wingless signaling in the Drosophila embryo: zygotic requirements and the role of the frizzled genes.

Wingless signaling plays a central role during epidermal patterning in Drosophila. We have analyzed zygotic requirements for Wingless signaling in the embryonic ectoderm by generating synthetic deficiencies that uncover more than 99% of the genome. We found no genes required for initial wingless expression, other than previously identified segmentation genes. In contrast, maintenance of wingless expression shows a high degree of zygotic transcriptional requirements. Besides known genes, we have identified at least two additional genomic regions containing new genes involved in Wingless maintenance. We also assayed for the zygotic requirements for Wingless response and found that no single genomic region was required for the cytoplasmic accumulation of Armadillo in the receiving cells. Surprisingly, embryos homozygously deleted for the candidate Wingless receptor, Dfrizzled2, showed a normal Wingless response. However, the Armadillo response to Wingless was strongly reduced in double mutants of both known members of the frizzled family in Drosophila, frizzled and Dfrizzled2. Based on their expression pattern during embryogenesis, different Frizzled receptors may play unique but overlapping roles in development. In particular, we suggest that Frizzled and Dfrizzled2 are both required for Wingless autoregulation, but might be dispensable for late Engrailed maintenance. While Wingless signaling in embryos mutant for frizzled and Dfrizzled2 is affected, Wingless protein is still internalized into cells adjacent to wingless-expressing cells. Incorporation of Wingless protein may therefore involve cell surface molecules in addition to the genetically defined signaling receptors of the frizzled family.

Regulation of armadillo by a Drosophila APC inhibits neuronal apoptosis during retinal development.

We find that inactivation of a Drosophila homolog of the tumor suppressor APC (D-APC) causes retinal neuronal degeneration and pigment cell hypertrophy, a phenotype remarkably similar to that found in humans with germline APC mutations. Retinal degeneration in the D-APC mutant results from apoptotic cell death, which accompanies a defect in neuronal differentiation. Reduction in the Drosophila beta-catenin, Armadillo (Arm), rescues the differentiation defect and prevents apoptosis in the D-APC mutant, while Arm overexpression mimics D-APC inactivation. A mutation in dTCF, the DNA-binding protein required in Arm-mediated signal transduction, can eliminate the cell death without rescuing the differentiation defect in D-APC mutants. Uncoupling of these two Arm-induced processes suggests a novel role for the Arm/dTCF complex in the activation of apoptosis.

Probing for gene specificity in epithelial development.

We surveyed a total of 228 random insertions of a P[GawB] element to determine the fraction of regulatory regions in the Drosophila genome that activate gene expression specifically in follicle cells versus producing more complex patterns of expression. We monitored the GAL4 expression encoded by this construct in the ovarian follicle cells by crossing the lines to a strain containing a lacZ reporter construct. Sixty four per cent of the insertions showed ovarian expression. To assess the specificity of this expression, 124 of the 228 lines were crossed to strains containing either an activated form of Armadillo, the Drosophila homolog of beta-catenin, or an activated form of Torpedo/Egfr, the Drosophila homolog of the Epidermal Growth Factor receptor, under the control of GAL4 target sites. The lethality and imaginal disc phenotypes observed in these crosses suggest that most random insertions cause GAL4 expression in a variety of tissues. Very few insertions appear to drive expression only in follicle cells. Although the activated form of Armadillo produced higher frequencies of lethality and disk phenotypes, expression in the follicle cell epithelium at later stages of oogenesis did not lead to a visible phenotype. This contrasts with the dorsalized phenotypes observed in the combination of the same GAL4 lines with the activated Torpedo construct.

Hyperactivation of the folded gastrulation pathway induces specific cell shape changes.

During Drosophila gastrulation, mesodermal precursors are brought into the interior of the embryo by formation of the ventral furrow. The first steps of ventral furrow formation involve a flattening of the apical surface of the presumptive mesodermal cells and a constriction of their apical diameters. In embryos mutant for folded gastrulation (fog), these cell shape changes occur but the timing and synchrony of the constrictions are abnormal. A similar phenotype is seen in a maternal effect mutant, concertina (cta). fog encodes a putative secreted protein whereas cta encodes an (alpha)-subunit of a heterotrimeric G protein. We have proposed that localized expression of the fog signaling protein induces apical constriction by interacting with a receptor whose downstream cellular effects are mediated by the cta G(alpha)protein.

In order to test this model, we have ectopically expressed fog at the blastoderm stage using an inducible promoter. In addition, we have examined the constitutive activation of cta protein by blocking GTP hydrolysis using both in vitro synthesized mutant alleles and cholera toxin treatment. Activation of the fog/cta pathway by any of these procedures results in ectopic cell shape changes in the gastrula. Uniform fog expression rescues the gastrulation defects of fog null embryos but not cta mutant embryos, arguing that cta functions downstream of fog expression. The normal location of the ventral furrow in embryos with uniformly expressed fog suggests the existence of a fog-independent pathway determining mesoderm-specific cell behaviors and invagination. Epistasis experiments indicate that this pathway requires snail but not twist expression.

Developmental regulation of vesicle transport in Drosophila embryos: forces and kinetics.

In Drosophila embryos, microtubules oriented along apical-basal directions support saltatory vesicle movement. Vesicle traffic includes lipid droplets whose distribution shifts twice during early embryogenesis. Using microscopy, optical tweezers, and a novel squashed-mount embryo preparation, we tracked single droplets and measured the forces these generated. Droplet stalling forces change developmentally, in a roughly quantized fashion, consistent with variation in the number of active motors. We characterized a mutation, klarsicht, that affects droplet transport. Klar+ facilitates changes in force, possibly by coordinating the activity of multiple motors. Alterations in transport affected motion in both apical and basal directions, indicating tight coupling between motors of opposite polarity. Mutations in klar also affect nuclear migration during eye development, suggesting multiple roles for klar-based transport.

Integration of the head and trunk segmentation systems controls cephalic furrow formation in Drosophila.

Genetic and molecular analyses of patterning of the Drosophila embryo have shown that the process of segmentation of the head is fundamentally different from the process of segmentation of the trunk. The cephalic furrow (CF), one of the first morphological manifestations of the patterning process, forms at the juxtaposition of these two patterning systems. We report here that the initial step in CF formation is a change in shape and apical positioning of a single row of cells. The anteroposterior position of these initiator cells may be defined by the overlapping expression of the head gap gene buttonhead (btd) and the primary pair-rule gene even-skipped (eve). Re-examination of the btd and eve phenotypes in live embryos indicated that both genes are required for CF formation. Further, Eve expression in initiator cells was found to be dependent upon btd activity. The control of eve expression by btd in these cells is the first indication of a new level of integrated regulation that interfaces the head and trunk segmentation systems. In conjunction with previous data on the btd and eve embryonic phenotypes, our results suggest that interaction between these two genes both controls initiation of a specific morphogenetic movement that separates two morphogenetic fields and contributes to patterning the hinge region that demarcates the procephalon from the segmented germ band.

A Drosophila homolog of the tumor suppressor gene adenomatous polyposis coli down-regulates beta-catenin but its zygotic expression is not essential for the regulation of Armadillo.

Mutations in the adenomatous polyposis coli gene (which encodes a protein called APC) are associated with the formation of intestinal polyps and colon cancers. To facilitate the functional study of APC we have isolated its Drosophila homolog (D-APC) by screening an expression library with an antibody against human APC. The isolated cDNA encodes a predicted 2416-amino acid protein containing significant homology to multiple domains of mammalian APCs. D-APC has seven complete armadillo repeats with 60% identity to its human homolog, one beta-catenin binding site, and up to 7 copies of a 20-amino acid repeat with the average of 50% identity to human APC at amino acid level. D-APC, like its human counterpart, also contains a basic domain. Expression of the domain of D-APC homologous to the region required for beta-catenin down-regulation resulted in down-regulation of intracellular beta-catenin in a mammalian cell line. This same region bound to the Armadillo (Arm) protein, in vitro, the Drosophila homolog of beta-catenin. D-APC RNA and protein expression is very low, if detectable at all, during stages when Arm protein accumulates in a striped pattern in the epidermis of the Drosophila embryos. Removing zygotic D-APC expression did not alter Arm protein distribution, and the final cuticle pattern was not affected significantly. As observed in the rodent, high levels of D-APC expression have been detected in the central nervous system, suggesting a role for D-APC in central nervous system formation.

armadillo, bazooka, and stardust are critical for early stages in formation of the zonula adherens and maintenance of the polarized blastoderm epithelium in Drosophila.

Cellularization of the Drosophila embryo results in the formation of a cell monolayer with many characteristics of a polarized epithelium. We have used antibodies specific to cellular junctions and nascent plasma membranes to study the formation of the zonula adherens (ZA) in relation to the establishment of basolateral membrane polarity. The same approach was then used as a test system to identify X-linked zygotically active genes required for ZA formation. We show that ZA formation begins during cellularization and that the basolateral membrane domain is established at mid-gastrulation. By creating deficiencies for defined regions of the X chromosome, we have identified genes that are required for the formation of the ZA and the generation of basolateral membrane polarity. We show that embryos mutant for both stardust (sdt) and bazooka (baz) fail to form a ZA. In addition to the failure to establish the ZA, the formation of the monolayered epithelium is disrupted after cellularization, resulting in formation of a multilayered cell sheet by mid-gastrulation. SEM analysis of mutant embryos revealed a conversion of cells exhibiting epithelial characteristics into cells exhibiting mesenchymal characteristics. To investigate how mutations that affect an integral component of the ZA itself influence ZA formation, we examined embryos with reduced maternal and zygotic supply of wild-type Arm protein. These embryos, like embryos mutant for both sdt and baz, exhibit an early disruption of ZA formation. These results suggest that early stages in the assembly of the ZA are critical for the stability of the polarized blastoderm epithelium.