Formation of morphogen gradients in the Drosophila wing.

Patterning of multicellular fields requires mechanisms to coordinate developmental decisions made by populations of cells. Evidence is accumulating that the necessary information is provided by localized sources of secreted signalling proteins which act as morphogens. We review evidence that Wingless, Dpp and Hedgehog proteins act as morphogens in the developing wing of Drosophila and discuss recent work illustrating that signalling helps to shape their activity gradients by regulating ligand distribution and by modulating the responsiveness of target cells. These studies suggest that there is more to being a morphogen than formation of a ligand gradient by passive diffusion.     

The interplay between morphogens and tissue growth

Morphogens are conserved, secreted signalling molecules that regulate the size, shape and patterning of animal tissues and organs. Recent experimental evidence has emphasized the fundamental role of tissue growth in expanding the expression domains of morphogens and their target genes, in generating morphogen gradients and in modulating the response of cells to morphogens. Moreover, the classic view of how morphogens, particularly through their concentration gradient, regulate tissue size during development has been revisited recently. In this review, we discuss how morphogens and tissue growth affect each other, and we attempt to integrate genetic and molecular evidence from vertebrate and invertebrate model systems to put forward the idea that the interaction between growth and morphogens is a general feature of highly proliferative tissues.     

The developmental roles of the extracellular matrix: beyond structure to regulation

Cells in multicellular organisms are surrounded by a complex three-dimensional macromolecular extracellular matrix (ECM). This matrix, traditionally thought to serve a structural function providing support and strength to cells within tissues, is increasingly being recognized as having pleiotropic effects in development and growth. Elucidation of the role that the ECM plays in developmental processes has been significantly advanced by studying the phenotypic and developmental consequences of specific genetic alterations of ECM components in the mouse. These studies have revealed the enormous contribution of the ECM to the regulation of key processes in morphogenesis and organogenesis, such as cell adhesion, proliferation, specification, migration, survival, and differentiation. The ECM interacts with signaling molecules and morphogens thereby modulating their activities. This review considers these advances in our understanding of the function of ECM proteins during development, extending beyond their structural capacity, to embrace their new roles in intercellula signaling.     

Morphogens: experimental illusion or reality?

The main attention is paid to the critical analysis of experimental data on morphogenetically active substances, so called "morphogens". It is proposed to consider the morphogens as specific transmitters providing for definite phases of morphogenetic tissues interactions, rather than as vectors of "morphogenetic information". In the normal development, the most studied morphogenetic tissue interactions can be referred to as so called permissive inductions, since the cells of the vertebrate embryos (amphibians, avians) are early determined for development in the ectomeso--and endodermal directions. A slow progress in studying the morphogens can be due to the following causes. 1. Theoretical "inadequacy" of the former concepts on the essence and mechanisms of embryonic induction. The necessity to develop a new system of concepts in this area of developmental biology is stressed. 2. Incompleteness of knowledge about the properties of reacting tissues and the mechanisms of action of morphogens. The early gastrula ectoderm of amphibians, most frequently used for testing the morphogens, appears to be a heterogenous population of the cells with different properties and potencies. It is, therefore, impossible to standardize strictly the biotesting of morphogens. It is suggested that the use to this end of aggregates of cell "strains" from the gastrula ectoderm, rather than of the gastrula ectoderm itself, may be more adequate 3. Insufficiency of embryonic material for biochemical identification and isolation of natural morphogens. A study of so called heterogenous inductors might be of help; these latter can be considered as analogs of natural morphogens. But the similarity of natural and heterogenous inductors can be limited only by their final effect on target tissue. The data are provided on the chemical nature, properties and mechanisms of action for a number of natural and heterogenous inductors (vegetalizing, neuralizing, mesodermalizing, lens-inducing factors). A conclusion is drawn that specific antigens do exist normally but they should not be established as a special class of "informationally important" molecules. The information necessary for development is contained in target cells and the function of a morphogen consists in providing for a definite link in the chain of processes leading to the switching on or expression of one or another programme. Only syntheses of specific proteins can, apparently, be programmed, thus reflecting the "onset" of differentiation path for a cell.     

Morphogens and endocytosis

During development, secreted signaling proteins of the Wingless/Wnt, Hedgehog and Decapentaplegic (Dpp)/Bone Morphogenic Protein (BMP) families act as morphogens. Previous work had shown that these molecules act directly on distant cells, although until recently nothing was known about how they reach those distant cells. During the past two years, work carried out on Drosophila using genetic and cell biology approaches have revealed that endocytosis plays a central part in the mechanisms that control the spread of morphogens.     

Morphogenetic messages are in the extracellular matrix: biotechnology from bench to bedside

The origin and evolution of multicellular metazoa was accompanied by the appearance of extracellular matrix. The demineralized extracellular matrix of bone is enriched in morphogenetic proteins that induce bone. Bone morphogenetic proteins (BMPs) are intimately bound to collagens. BMP-4 has high affinity for type-IV collagen, and other binding proteins such as noggin and chordin. Soluble morphogens are kept in the solid state by extracellular matrix. In this sense Nature used the principles of affinity matrices long before humans patented the principle of affinity chromatography.     

Release and trafficking of lipid-linked morphogens

Wnt and Hedgehog family proteins are secreted morphogens that act on surrounding cells to pattern many different tissues in both vertebrates and invertebrates. The discovery that these proteins are covalently linked to lipids has raised the puzzling problem of how they come to be released from cells and move through tissue. A synergistic combination of biochemical, cell biological and genetic approaches over the past several years is beginning to illuminate both the forms in which lipid-linked morphogens are released from cells and the variety of molecular and cell biological mechanisms that control their dispersal.     

Longitudinal axon guidance

Our knowledge about molecular mechanisms underlying axon guidance along the antero-posterior axis in contrast to the dorso-ventral axis of the developing nervous system is very limited. During the past two years in vitro and in vivo studies have indicated that morphogens have a role in longitudinal axon guidance. Morphogens are secreted proteins that act in a concentration-dependent manner on susceptible groups of precursor cells and induce their differentiation to a specific cell fate. Thus, gradients of morphogens are responsible for the appropriate patterning of the nervous system during early phases of neural development. Therefore, it was surprising to find that gradients of two of these morphogens, Wnt4 and Shh, can be re-used for longitudinal axon guidance during later stages of nervous system development.     

Novel brain wiring functions for classical morphogens: a role as graded positional cues in axon guidance

During embryonic development, morphogens act as graded positional cues to dictate cell fate specification and tissue patterning. Recent findings indicate that morphogen gradients also serve to guide axonal pathfinding during development of the nervous system. These findings challenge our previous notions about morphogens and axon guidance molecules, and suggest that these proteins, rather than having sharply divergent functions, act more globally to provide graded positional information that can be interpreted by responding cells either to specify cell fate or to direct axonal pathfinding. This review presents the roles identified for members of three prominent morphogen families--the Hedgehog, Wnt and TGFbeta/BMP families--in axon guidance, and discusses potential implications for the molecular mechanisms underlying their guidance functions.     

Discrete interactions in cell adhesion measured by single-molecule force spectroscopy

Cell-cell adhesion mediated by specific cell-surface molecules is essential for multicellular development. Here we quantify de-adhesion forces at the resolution of individual cell-adhesion molecules, by controlling the interactions between single cells and combining single-molecule force spectroscopy with genetic manipulation. Our measurements are focused on a glycoprotein, contact site A (csA), as a prototype of cell-adhesion proteins. csA is expressed in aggregating cells of Dictyostelium discoideum, which are engaged in development of a multicellular organism. Adhesion between two adjacent cell surfaces involves discrete interactions characterized by an unbinding force of 23 +/- 8 pN, measured at a rupture rate of 2.5 +/- 0.5 microm s-1.     

Three Drosophila EXT genes shape morphogen gradients through synthesis of heparan sulfate proteoglycans

The signaling molecules Hedgehog (Hh), Decapentaplegic (Dpp) and Wingless (Wg) function as morphogens and organize wing patterning in Drosophila. In the screen for mutations that alter the morphogen activity, we identified novel mutants of two Drosophila genes, sister of tout-velu (sotv) and brother of tout-velu (botv), and new alleles of tout-velu (ttv). The encoded proteins of these genes belong to an EXT family of proteins that have or are closely related to glycosyltransferase activities required for biosynthesis of heparan sulfate proteoglycans (HSPGs). Mutation in any of these genes impaired biosynthesis of HSPGs in vivo, indicating that, despite their structural similarity, they are not redundant in the HSPG biosynthesis. Protein levels and signaling activities of Hh, Dpp and Wg were reduced in the cells mutant for any of these EXT genes to a various degree, Wg signaling being the least sensitive. Moreover, all three morphogens were accumulated in the front of EXT mutant cells, suggesting that these morphogens require HSPGs to move efficiently. In contrast to previous reports that ttv is involved exclusively in Hh signaling, we found that ttv mutations also affected Dpp and Wg. These data led us to conclude that each of three EXT genes studied contribute to Hh, Dpp and Wg morphogen signaling. We propose that HSPGs facilitate the spreading of morphogens and therefore, function to generate morphogen concentration gradients.     

Morphogen gradients. A question of time or concentration?

Morphogens are secreted proteins that organize surrounding tissues into distinct territories and are thought to act as a function of a threshold of their concentration. Although it has been demonstrated that morphogens act directly on the cells and do not rely on secondary signalling relays, intracellular sequential induction mechanisms, which are dependent on a simple signalling instruction, have not been excluded. Here, we present an alternative model to account for the organizing properties of morphogens, and propose that initial exposure to signalling changes cell context, which in combination with continuing morphogen activity, results in the expression of novel targets.     

Brain-derived neurotrophic factor is an anterograde survival factor in the rat visual system

BACKGROUND: The neurotrophins, which include nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), NT-4/5 and NT-6, are a family of proteins that play fundamental roles in the differentiation, survival and maintenance of peripheral and central neurons. Much research has focused on the role of neurotrophins as target-derived, retrogradely transported trophic molecules. Although there is recent evidence that BDNF and NT-3 can be transported in an anterograde direction along peripheral and central axons, there is as yet no conclusive evidence that these anterograde factors have direct post-synaptic actions. RESULTS: We report that BDNF travels in an anterograde direction along the optic nerve. The anterogradely transported BDNF had rapid effects on retinal target neurons in the superior colliculus and lateral geniculate nucleus of the brain. When endogenous BDNF within the developing superior colliculus was neutralised, the rate of programmed neuronal death increased. Conversely, provision of an afferent supply of BDNF prevented the degeneration of geniculate neurons after removal of their cortical target. CONCLUSIONS: BDNF released from retinal ganglion cells acts as a survival factor for post-synaptic neurons in retinal target fields.     

Patterns of mesenchymal condensation in a multiscale, discrete stochastic model

Cells of the embryonic vertebrate limb in high-density culture undergo chondrogenic pattern formation, which results in the production of regularly spaced “islands” of cartilage similar to the cartilage primordia of the developing limb skeleton. The first step in this process, in vitro and in vivo, is the generation of “cell condensations,” in which the precartilage cells become more tightly packed at the sites at which cartilage will form. In this paper we describe a discrete, stochastic model for the behavior of limb bud precartilage mesenchymal cells in vitro. The model uses a biologically motivated reaction–diffusion process and cell-matrix adhesion (haptotaxis) as the bases of chondrogenic pattern formation, whereby the biochemically distinct condensing cells, as well as the size, number, and arrangement of the multicellular condensations, are generated in a self-organizing fashion. Improving on an earlier lattice-gas representation of the same process, it is multiscale (i.e., cell and molecular dynamics occur on distinct scales), and the cells are represented as spatially extended objects that can change their shape. The authors calibrate the model using experimental data and study sensitivity to changes in key parameters. The simulations have disclosed two distinct dynamic regimes for pattern self-organization involving transient or stationary inductive patterns of morphogens. The authors discuss these modes of pattern formation in relation to available experimental evidence for the in vitro system, as well as their implications for understanding limb skeletal patterning during embryonic development.     

Short-range cell interactions and cell survival in the Drosophila wing

During development of multicellular organisms, cells are often eliminated by apoptosis if they fail to receive appropriate signals from their surroundings. Here, we report on short-range cell interactions that support cell survival in the Drosophila wing imaginal disc. We present evidence showing that cells incorrectly specified for their position undergo apoptosis because they fail to express specific proteins that are found on surrounding cells, including the LRR transmembrane proteins Capricious and Tartan. Interestingly, only the extracellular domains of Capricious and Tartan are required, suggesting that a bidirectional process of cell communication is involved in triggering apoptosis. We also present evidence showing that activation of the Notch signal transduction pathway is involved in triggering apoptosis of cells misspecified for their dorsal-ventral position.     

Towards the identification of the common features of bacterial biofilm development.

Microorganisms can live and proliferate as individual cells swimming freely in the environment, or they can grow as highly organized, multicellular communities encased in a self-produced polymeric matrix in close association with surfaces and interfaces. This microbial lifestyle is referred to as biofilms. The intense search over the last few years for factors involved in biofilm development has revealed that distantly related bacterial species recurrently make use of the same elements to produce biofilms. These common elements include a group of proteins containing GGDEF/EAL domains, surface proteins homologous to Bap of Staphylococcus aureus, and some types of exopolysaccharides, such as cellulose and the poly-beta-1,6-N-acetylglucosamine. This review summarizes current knowledge about these three common elements and their role in biofilm development.     

Comparative proteomic analysis of Helicoverpa armigera cells undergoing apoptosis

Apoptosis is of crucial importance in the life of multicellular organisms. In holometabolous insects, particularly in Lepidoptera, apoptosis is essential in biological processes such as metamorphosis and defense against pathogens. Apoptosis is tightly regulated and involves many proteins, among them caspases, which play a central role. In mammals, almost 300 targets of caspases have been described, and the expression of more than a hundred proteins has been shown to be altered in apoptotic cells. To date, the molecular pathways controlling apoptosis are poorly understood in Lepidoptera. Here, we used a comparative approach aiming to identify candidate proteins potentially implicated in these pathways. We examined changes occurring, in the proteome of a Helicoverpa armigera-derived cell line, upon induction by actinomycin D. We identified 13 proteins for which the relative abundance was significantly altered. Among these, the abundance of procaspase-1 decreased in apoptotic cells, reflecting its processing into the active form. We characterized its properties by heterologous expression and correlated the observed substrate specificity with changes in caspase activity in HaAM1 cells after induction. We also identified three chaperones as well as several putative pro- and anti-apoptotic proteins. Altogether, these data suggest that apoptotic pathways in Lepidoptera share similarities with the ones described in mammals.