Boundaries in development: formation and function

The odd-skipped family of zinc finger genes promotes Drosophila leg segmentation

Notch signaling controls formation of joints at leg segment borders and growth of the developing Drosophila leg. Here, we identify the odd-skipped gene family as a key group of genes that function downstream of the Notch receptor to promote morphological changes associated with joint formation during leg development. odd, sob, drm, and bowl are expressed in a segmental pattern in the developing leg, and their expression is regulated by Notch signaling. Ectopic expression of odd, sob, or drm can induce invaginations in the leg disc epithelium and morphological changes in the adult leg that are characteristic of endogenous invaginating joint cells. These effects are not due to an alteration in the expression of other genes of the developing joint. While odd or drm mutant clones do not affect leg segmentation, and thus appear to act redundantly, bowl mutant clones do perturb leg development. Specifically, bowl mutant clones result in a failure of joint formation from the distal tibia to tarsal segment 5, while more proximal clones cause melanotic protrusions from the leg cuticle. Together, these results indicate that the odd-skipped family of genes mediates Notch function during leg development by promoting a specific aspect of joint formation, an epithelial invagination. As the odd-skipped family genes are involved in regulating cellular morphogenesis during both embryonic segmentation and hindgut development, we suggest that they may be required in multiple developmental contexts to induce epithelial cellular changes.

Glycosylation regulates Notch signalling

Intracellular post-translational modifications such as phosphorylation and ubiquitylation have been well studied for their roles in regulating diverse signalling pathways, but we are only just beginning to understand how differential glycosylation is used to regulate intercellular signalling. Recent studies make clear that extracellular post-translational modifications, in the form of glycosylation, are essential for the Notch signalling pathway, and that differences in the extent of glycosylation are a significant mechanism by which this pathway is regulated.

Notch signaling in the immune system

Notch signaling plays a preeminent role during development in not only regulating cell fate decisions, but it can also influence growth and survival of progenitor cells. In the immune system, Notch is required for the maintenance of hematopoietic stem cells and in directing T- versus B-lineage commitment. In this review, I will summarize some of the recent findings relating to the function of Notch in the immune system during lymphocyte development and in the generation and function of mature cells.

Examination of the relationship between astrocyte morphology and laminar boundaries in the molecular layer of adult dentate gyrus

Astrocytes are known to play an integral role in the development of compartmental boundaries in the brain and in the creation of trauma-induced boundaries. However, the physical relationship between astrocytes and such boundaries in the adult brain is less clear. If astrocytes do respect or play an ongoing role in maintaining such boundaries, a correlation between the position of such a boundary and the morphology of neighboring astrocytes might be observable. In this study, we examined the distribution of astrocytes with respect to the laminar boundaries compartmentalizing afferents to the dentate gyrus molecular layer. In addition, we attempted to determine whether astrocyte morphology is influenced by these laminar boundaries. To this end, protoplasmic astrocytes in the adult rat dentate gyrus were revealed with fluorescent tracer dyes and subsequently analyzed with respect to laminar boundaries demarcated by means of immunolabeling for the lamina-specific molecules EphA4 and neural cell adhesion molecule (N-CAM). We find that astrocyte distribution is influenced by the boundary separating the associational/commissural and perforant path afferents. In addition, we show that astrocytes in this region are polarized in their morphology, unlike typically stellate astrocytes, but that the laminar boundaries themselves do not appear to confer this morphology. This polarized morphology, however acquired, may have import for the functioning of astrocytes within the highly organized composition of the dentate gyrus molecular layer and for the overall microphysiology of this and other brain regions.

Specification of cell fates within the salivary gland primordium

The Drosophila salivary gland is a simple tubular organ derived from a contiguous epithelial primordium, which is established by the activities of the homeodomain-containing proteins Sex combs reduced (SCR), Extradenticle (EXD), and Homothorax (HTH). EGF signaling along the ventral midline specifies the salivary duct fate for cells in the center of the primordium, while cells farther away from the source of EGF signal adopt a secretory cell fate. EGF signaling works, at least in part, by repressing expression of secretory cell genes in the duct primordium, including fork head (fkh), which encodes a winged-helix transcription factor. FKH, in turn, represses trachealess (trh), a duct-specific gene initially expressed throughout the salivary gland primordium. trh encodes a basic helix-loop-helix PAS-domain containing transcription factor that has been proposed to specify the salivary duct fate. In conflict with this model, we find that three genes, dead ringer (dri), Serrate (Ser), and trh itself, are expressed in the duct independently of trh. Expression of all three duct genes is repressed in the secretory cells by FKH. We also show that SER in the duct cells signals to the adjacent secretory cells to specify a third cell type, the imaginal ring cells. Thus, localized EGF- and Notch-signaling transform a uniform epithelial sheet into three distinct cell types. In addition, Ser directs formation of actin rings in the salivary duct.

Molecular genetic analysis of the glycosyltransferase Fringe in Drosophila

Fringe proteins are beta1,3-N-acetylglucosaminyltransferases that modulate signaling through Notch receptors by modifying O-linked fucose on epidermal growth factor domains. Fringe is highly conserved, and comparison among 18 different Fringe proteins from 11 different species identifies a core set of 84 amino acids that are identical among all Fringes. Fringe is only distantly related to other glycosyltransferases, but analysis of the predicted Drosophila proteome identifies a set of four sequence motifs shared among Fringe and other putative beta1,3-glycosyltransferases. To gain functional insight into these conserved sequences, we genetically and molecularly characterized 14 point mutations in Drosophila fringe. Most nonsense mutations act as recessive antimorphs, raising the possibility that Fringe may function as a dimer. Missense mutations identify two distinct motifs that are conserved among beta1,3-glycosyltransferases, and that can be modeled onto key motifs in the crystallographic structures of bovine beta1,4-galactosyltransferase 1 and human glucuronyltransferase I. Other missense mutations map to amino acids that are conserved among Fringe proteins, but not among other glycosyltransferases, and thus may identify structural motifs that are required for unique aspects of Fringe activity.

Multiple joint and skeletal patterning defects caused by single and double mutations in the mouse Gdf6 and Gdf5 genes

Growth/differentiation factors 5, 6, and 7 (GDF5/6/7) represent a distinct subgroup within the bone morphogenetic protein (BMP) family of secreted signaling molecules. Previous studies have shown that the Gdf5 gene is expressed in transverse stripes across developing skeletal elements and is one of the earliest known markers of joint formation during embryonic development. Although null mutations in this gene disrupt formation of some bones and joints in the skeleton, many sites are unaffected. Here, we show that the closely related family members Gdf6 and Gdf7 are expressed in different subsets of developing joints. Inactivation of the Gdf6 gene causes defects in joint, ligament, and cartilage formation at sites distinct from those seen in Gdf5 mutants, including the wrist and ankle, the middle ear, and the coronal suture between bones in the skull. Mice lacking both Gdf5 and Gdf6 show additional defects, including severe reduction or loss of some skeletal elements in the limb, additional fusions between skeletal structures, scoliosis, and altered cartilage in the intervertebral joints of the spinal column. These results show that members of the GDF5/6/7 subgroup are required for normal formation of bones and joints in the limbs, skull, and axial skeleton. The diverse effects on joint development and the different types of joints affected in the mutants suggest that members of the GDF family play a key role in establishing boundaries between many different skeletal elements during normal development. Some of the skeletal defects seen in single or double mutant mice resemble defects seen in human skeletal diseases, which suggests that these genes may be candidates that underlie some forms of carpal/tarsal coalition, conductive deafness, scoliosis, and craniosynostosis.

Origins of differentiation via phenotypic plasticity

How cell types of multicellular organisms came to be differentiated is still an open issue. Here I offer a model that posits that the origins of some cell differentiation patterns were originally passive outcomes of environmental effects. As cells' contact with the external environment was diminished, their patterns of gene expression were altered, due to changes in concentrations of externally supplied substances. Later, as multicellular growth continued, the relationships of cell layers to each other shifted, producing concentration gradients of signaling molecules. These gradients emanated both from the external cell layer toward the inside and from internal cell layers to adjacent layers. In this scenario then, differentiation arose initially as a by-product of the changing patterns of gene expression and of the complex mixtures and changing concentrations of substances passing among layers. Subsequent selection would operate to stabilize the expression patterns in those cell layers whose phenotypes provide a fitness advantage to the organism.

Node and midline defects are associated with left-right development in Delta1 mutant embryos

Axes formation is a fundamental process of early embryonic development. In addition to the anteroposterior and dorsoventral axes, the determination of the left-right axis is crucial for the proper morphogenesis of internal organs and is evolutionarily conserved in vertebrates. Genes known to be required for the normal establishment and/or maintenance of left-right asymmetry in vertebrates include, for example, components of the TGF-beta family of intercellular signalling molecules and genes required for node and midline function. We report that Notch signalling, which previously had not been implicated in this morphogenetic process, is required for normal left-right determination in mice. We show, that the loss-of-function of the delta 1 (Dll1) gene causes a situs ambiguous phenotype, including randomisation of the direction of heart looping and embryonic turning. The most probable cause for this left-right defect in Dll1 mutant embryos is a failure in the development of proper midline structures. These originate from the node, which is disrupted and deformed in Dll1 mutant embryos. Based on expression analysis in wild-type and mutant embryos, we suggest a model, in which Notch signalling is required for the proper differentiation of node cells and node morphology.

Regulation of notch signaling by o-linked fucose

Notch and its ligands are modified by a protein O-fucosyltransferase (OFUT1) that attaches fucose to a Serine or Threonine within EGF domains. By using RNAi to decrease Ofut1 expression in Drosophila, we demonstrate that O-linked fucose is positively required for Notch signaling, including both Fringe-dependent and Fringe-independent processes. The requirement for Ofut1 is cell autonomous, in the signal-receiving cell, and upstream of Notch activation. The transcription of Ofut1 is developmentally regulated, and surprisingly, overexpression of Ofut1 inhibits Notch signaling. Together, these results indicate that OFUT1 is a core component of the Notch pathway, which is required for the activation of Notch by its ligands, and whose regulation may contribute to the pattern of Notch activation during development.

Morphogen gradients, positional information, and Xenopus: interplay of theory and experiment

The idea of morphogen gradients has long been an important one in developmental biology. Studies with amphibians and with Xenopus in particular have made significant contributions to demonstrating the existence, identity, and mechanisms of action of morphogens. Mesoderm induction and patterning by activin, nodals, bone morphogenetic proteins, and fibroblast growth factors have been analyzed thoroughly and reveal recurrent and combinatorial roles for these protein growth factor morphogens and their antagonists. The dynamics of nodal-type signaling and the intersection of VegT and beta-catenin intracellular gradients reveal detailed steps in early long-range patterning. Interpretation of gradients requires sophisticated mechanisms for sharpening thresholds, and the activin-Xbra-Gsc system provides an example of this. The understanding of growth factor signal transduction has elucidated growth factor morphogen action and provided tools for dissecting their direct long-range action and distribution. The physical mechanisms of morphogen gradient establishment are the focus of new interest at both the experimental and theoretical level. General themes and emerging trends in morphogen gradient studies are discussed.

Organizer activity of the polar cells duringDrosophilaoogenesis

Patterning of the Drosophila egg requires the establishment of several distinct types of somatic follicle cells, as well as interactions between these follicle cells and the oocyte. The polar cells occupy the termini of the follicle and are specified by the activation of Notch. We have investigated their role in follicle patterning by creating clones of cells mutant for the Notch modulator fringe. This genetic ablation of polar cells results in cell fate defects within surrounding follicle cells. At the anterior, the border cells, the immediately adjacent follicle cell fate, are absent, as are the more distant stretched and centripetal follicle cells. Conversely, increasing the number of polar cells by expressing an activated form of the Notch receptor increases the number of border cells. At the posterior, elimination of polar cells results in abnormal oocyte localization. Moreover, when polar cells are mislocalized laterally, the surrounding follicle cells adopt a posterior fate, the oocyte is located adjacent to them,and the anteroposterior axis of the oocyte is re-oriented with respect to the ectopic polar cells. Our observations demonstrate that the polar cells act as an organizer that patterns surrounding follicle cells and establishes the anteroposterior axis of the oocyte. The origin of asymmetry duringDrosophila development can thus be traced back to the specification of the polar cells during early oogenesis.

Morphological boundary forms by a novel inductive event mediated by Lunatic fringe and Notch during somitic segmentation

Boundary formation plays a central role in differentiating the flanking regions that give rise to discrete tissues and organs during early development. We have studied mechanisms by which a morphological boundary and tissue separation are regulated by examining chicken somite segmentation as a model system. By transplanting a small group of cells taken from a presumptive border into a non-segmentation site, we have found a novel inductive event where posteriorly juxtaposed cells to the next-forming border instruct the anterior cells to become separated and epithelialized. We have further studied the molecular mechanisms underlying these interactions by focusing on Lunatic fringe, a modulator of Notch signaling, which is expressed in the region of the presumptive boundary. By combining DNA in ovo electroporation and embryonic transplantation techniques we have ectopically made a sharp boundary of Lunatic fringe activity in the unsegmented paraxial mesoderm and observed a fissure formed at the interface. In addition, a constitutive active form of Notch mimics this instructive phenomenon. These suggest that the boundary-forming signals emanating from the posterior border cells are mediated by Notch, the action of which is confined to the border region by Lunatic fringe within the area where mRNAs of Notch and its ligand are broadly expressed in the presomitic mesoderm.

The establishment of segmentation in the Drosophila leg

Segmentation is a developmental mechanism that subdivides a tissue into repeating functional units, which can then be further elaborated upon during development. In contrast to embryonic segmentation, Drosophila leg segmentation occurs in a tissue that is rapidly growing in size and thus segmentation must be coordinated with tissue growth. I demonstrate that segmentation of the Drosophila leg, as assayed by expression of the key regulators of segmentation, the Notch ligands and fringe, occurs progressively and I define the sequence in which the initial segmental subdivisions arise. I further demonstrate that the proximal-distal patterning genes homothorax and dachshund are positively required, while Distal-less is unexpectedly negatively required, to establish the segmental pattern of Notch ligand and fringe expression. Two Serrate enhancers that respond to regulation by dachshund are also identified. Together, these studies provide evidence that distinct combinations of the proximal-distal patterning genes independently regulate each segmental ring of Notch ligand and fringe expression and that this regulation occurs through distinct enhancers. These studies thus provide a molecular framework for understanding how segmentation during tissue growth is accomplished.