Morphogen transport along epithelia, an integrated trafficking problem

Hedgehog morphogen gradient is robust towards variations in tissue morphology in Drosophila

During tissue development, gradients of secreted signaling molecules known as morphogens provide cells with positional information. The mechanisms underlying morphogen spreading have been widely studied, however, it remains largely unexplored whether the shape of morphogen gradients is influenced by tissue morphology. Here, we developed an analysis pipeline to quantify the distribution of proteins within a curved tissue. We applied it to the Hedgehog morphogen gradient in the Drosophila wing and eye-antennal imaginal discs, which are flat and curved tissues, respectively. Despite a different expression profile, the slope of the Hedgehog gradient was comparable between the two tissues. Moreover, inducing ectopic folds in wing imaginal discs did not affect the slope of the Hedgehog gradient. Suppressing curvature in the eye-antennal imaginal disc also did not alter the Hedgehog gradient slope but led to ectopic Hedgehog expression. In conclusion, through the development of an analysis pipeline that allows quantifying protein distribution in curved tissues, we show that the Hedgehog gradient is robust towards variations in tissue morphology.

Interplay between morphogen-directed positional information systems and physiological signaling

The development of an organism from an undifferentiated single cell into a spatially complex structure requires spatial patterning of cell fates across tissues. Positional information, proposed by Lewis Wolpert in 1969, has led to the characterization of many components involved in regulating morphogen signaling activity. However, how morphogen gradients are established, maintained, and interpreted by cells still is not fully understood. Quantitative and systems-based approaches are increasingly needed to define general biological design rules that govern positional information systems in developing organisms. This short review highlights a selective set of studies that have investigated the roles of physiological signaling in modulating and mediating morphogen-based pattern formation. Similarities between neural transmission and morphogen-based pattern formation mechanisms suggest underlying shared principles of active cell-based communication. Within larger tissues, neural networks provide directed information, via physiological signaling, that supplements positional information through diffusion. Further, mounting evidence demonstrates that physiological signaling plays a role in ensuring robustness of morphogen-based signaling. We conclude by highlighting several outstanding questions regarding the role of physiological signaling in morphogen-based pattern formation. Elucidating how physiological signaling impacts positional information is critical for understanding the close coupling of developmental and cellular processes in the context of development, disease, and regeneration.

WNT signalling events near the cell membrane and their pharmacological targeting for the treatment of cancer

WNT signalling is an essential signalling pathway for all multicellular animals. Although first described more than 30 years ago, new components and regulators of the pathway are still being discovered. Considering its importance in both embryonic development and adult homeostasis, it is not surprising that this pathway is often deregulated in human diseases such as cancer. Recently, it became clear that in addition to cytoplasmic components such as β-catenin, other, membrane-bound or extracellular, components of the WNT pathway are also altered in cancer. This review gives an overview of the recent discoveries on WNT signalling events near the cell membrane. Furthermore, membrane-associated components of the WNT pathway, which are more accessible for therapeutic intervention, as well therapeutic approaches that already target those components will be discussed. In this way, we hope to stimulate the development of effective anti-cancer therapies that target this fascinating pathway.

LINKED ARTICLES

This article is part of a themed section on WNT Signalling: Mechanisms and Therapeutic Opportunities. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.24/issuetoc.

The role of mathematical models in understanding pattern formation in developmental biology

In a Wall Street Journal article published on April 5, 2013, E. O. Wilson attempted to make the case that biologists do not really need to learn any mathematics-whenever they run into difficulty with numerical issues, they can find a technician (aka mathematician) to help them out of their difficulty. He formalizes this in Wilsons Principle No. 1: "It is far easier for scientists to acquire needed collaboration from mathematicians and statisticians than it is for mathematicians and statisticians to find scientists able to make use of their equations." This reflects a complete misunderstanding of the role of mathematics in all sciences throughout history. To Wilson, mathematics is mere number crunching, but as Galileo said long ago, "The laws of Nature are written in the language of mathematics[Formula: see text] the symbols are triangles, circles and other geometrical figures, without whose help it is impossible to comprehend a single word." Mathematics has moved beyond the geometry-based model of Galileo's time, and in a rebuttal to Wilson, E. Frenkel has pointed out the role of mathematics in synthesizing the general principles in science (Both point and counter-point are available in Wilson and Frenkel in Notices Am Math Soc 60(7):837-838, 2013). We will take this a step further and show how mathematics has been used to make new and experimentally verified discoveries in developmental biology and how mathematics is essential for understanding a problem that has puzzled experimentalists for decades-that of how organisms can scale in size. Mathematical analysis alone cannot "solve" these problems since the validation lies at the molecular level, but conversely, a growing number of questions in biology cannot be solved without mathematical analysis and modeling. Herein, we discuss a few examples of the productive intercourse between mathematics and biology.

ROBUSTNESS OF MORPHOGEN GRADIENTS WITH "BUCKET BRIGADE" TRANSPORT THROUGH MEMBRANE-ASSOCIATED NON-RECEPTORS

Robust multiple-fate morphogen gradients are essential for embryo development. Here, we analyze mathematically a model of morphogen gradient (such as Dpp in Drosophila wing imaginal disc) formation in the presence of non-receptors with both diffusion of free morphogens and the movement of morphogens bound to non-receptors. Under the assumption of rapid degradation of unbound morphogen, we introduce a method of functional boundary value problem and prove the existence, uniqueness and linear stability of a biologically acceptable steady-state solution. Next, we investigate the robustness of this steady-state solution with respect to significant changes in the morphogen synthesis rate. We prove that the model is able to produce robust biological morphogen gradients when production and degradation rates of morphogens are large enough and non-receptors are abundant. Our results provide mathematical and biological insight to a mechanism of achieving stable robust long distance morphogen gradients. Key elements of this mechanism are rapid turnover of morphogen to non-receptors of neighoring cells resulting in significant degradation and transport of non-receptor-morphogen complexes, the latter moving downstream through a "bucket brigade" process.

SNX3 controls Wingless/Wnt secretion through regulating retromer-dependent recycling of Wntless

Drosophila Wingless (Wg) acts as a morphogen during development. Wg secretion is controlled by a seven-pass transmembrane cargo Wntless (Wls). We have recently identified retromer as a key regulator involved in Wls trafficking. As sorting nexin (SNX) molecules are essential components of the retromer complex, we hypothesized that specific SNX(s) is required for retromer-mediated Wnt secretion. Here, we generated Drosophila mutants for all of the eight snx members, and identified Drosophila SNX3 (DSNX3) as an essential molecule required for Wg secretion. We show that Wg secretion and its signaling activity are defective in Dsnx3 mutant clones in wing discs. Wg levels in the culture medium of Dsnx3-depleted S2 cells are also markedly reduced. Importantly, Wls levels are strikingly reduced in Dsnx3 mutant cells, and overexpression of Wls can rescue the Wg secretion defect observed in Dsnx3 mutant cells. Moreover, DSNX3 can interact with the retromer component Vps35, and co-localize with Vps35 in early endosomes. These data indicate that DSNX3 regulates Wg secretion via retromer-dependent Wls recycling. In contrast, we found that Wg secretion is not defective in cells mutant for Drosophila snx1 and snx6, two components of the classical retromer complex. Ectopic expression of DSNX1 or DSNX6 fails to rescue the Wg secretion defect in Dsnx3 mutant wing discs and in Dsnx3 dsRNA-treated S2 cells. These data demonstrate the specificity of the DSNX3-retromer complex in Wls recycling. Together, our findings suggest that DSNX3 acts as a cargo-specific component of retromer, which is required for endocytic recycling of Wls and Wg/Wnt secretion.

Extracellular movement of signaling molecules

Extracellular signaling molecules have crucial roles in development and homeostasis, and their incorrect deployment can lead to developmental defects and disease states. Signaling molecules are released from sending cells, travel to target cells, and act over length scales of several orders of magnitude, from morphogen-mediated patterning of small developmental fields to hormonal signaling throughout the organism. We discuss how signals are modified and assembled for transport, which routes they take to reach their targets, and how their range is affected by mobility and stability.

Heparin-like heparan sulfate from rabbit cartilage

Glycosaminoglycans were extracted from both young rabbit growth plate (GRP) and articular (ART) cartilage tissues and enzymatically treated to selectively eliminate chondroitin sulfates and hyaluronic acid. The procedure avoided any fractionation step that could enrich the extract with over- or under-sulfated species. Isolated heparan sulfate (HS) was characterized by mono- and bidimensional nuclear magnetic resonance (NMR) spectroscopy to quantify their specific structural features and/or by mass spectrometry to establish the disaccharide composition. Both GRP and ART HSs, despite differing in their yield (GRP at least 100 times greater than ART), exhibited a surprisingly high degree of sulfation. Quantitative two-dimensional heteronuclear single-quantum coherence-NMR analysis of GRP HS revealed unusually high N-sulfated glucosamine and 2-O-sulfated iduronic acid contents, similar to heparin. The unique pentasaccharide sequence of the binding site for antithrombin was also detected in a significant amount. High-performance liquid chromatography mass spectrometry analysis of the enzymatic digests with a cocktail of heparin lyases of both cartilaginous HSs confirmed the NMR results. As well as the discovery of an unusual HS structure in the two different types of rabbit cartilage, the feasibility of the analytical method adopted here has been demonstrated within this study. Such a method can be used to isolate and analyze HS from both normal and pathologic tissues. Characterization of healthy and pathological HS structures will contribute to improve the understanding of diseases related to malfunctions of HS biosynthesis and/or metabolism.

Morphogen gradients: from generation to interpretation

Morphogens are long-range signaling molecules that pattern developing tissues in a concentration-dependent manner. The graded activity of morphogens within tissues exposes cells to different signal levels and leads to region-specific transcriptional responses and cell fates. In its simplest incarnation, a morphogen signal forms a gradient by diffusion from a local source and clearance in surrounding tissues. Responding cells often transduce morphogen levels in a linear fashion, which results in the graded activation of transcriptional effectors. The concentration-dependent expression of morphogen target genes is achieved by their different binding affinities for transcriptional effectors as well as inputs from other transcriptional regulators. Morphogen distribution and interpretation are the result of complex interactions between the morphogen and responding tissues. The response to a morphogen is dependent not simply on morphogen concentration but also on the duration of morphogen exposure and the state of the target cells. In this review, we describe the morphogen concept and discuss the mechanisms that underlie the generation, modulation, and interpretation of morphogen gradients.

Formation of the long range Dpp morphogen gradient

The TGF-β homolog Decapentaplegic (Dpp) acts as a secreted morphogen in the Drosophila wing disc, and spreads through the target tissue in order to form a long range concentration gradient. Despite extensive studies, the mechanism by which the Dpp gradient is formed remains controversial. Two opposing mechanisms have been proposed: receptor-mediated transcytosis (RMT) and restricted extracellular diffusion (RED). In these scenarios the receptor for Dpp plays different roles. In the RMT model it is essential for endocytosis, re-secretion, and thus transport of Dpp, whereas in the RED model it merely modulates Dpp distribution by binding it at the cell surface for internalization and subsequent degradation. Here we analyzed the effect of receptor mutant clones on the Dpp profile in quantitative mathematical models representing transport by either RMT or RED. We then, using novel genetic tools, experimentally monitored the actual Dpp gradient in wing discs containing receptor gain-of-function and loss-of-function clones. Gain-of-function clones reveal that Dpp binds in vivo strongly to the type I receptor Thick veins, but not to the type II receptor Punt. Importantly, results with the loss-of-function clones then refute the RMT model for Dpp gradient formation, while supporting the RED model in which the majority of Dpp is not bound to Thick veins. Together our results show that receptor-mediated transcytosis cannot account for Dpp gradient formation, and support restricted extracellular diffusion as the main mechanism for Dpp dispersal. The properties of this mechanism, in which only a minority of Dpp is receptor-bound, may facilitate long-range distribution.

Morphogens are signaling molecules that trigger specific responses in cells in a concentration-dependent manner. The formation of morphogen gradients is essential for the patterning of tissues and organs. Decapentaplegic (Dpp) is the Drosophila homolog of the bone morphogenic proteins in vertebrates and forms a morphogen gradient along the anterior-posterior axis of the Drosophila wing imaginal disc, a single-cell layered epithelium. Dpp determines the growth and final size of the wing disc and serves as an ideal model system to study gradient formation. Despite extensive studies the mechanism by which morphogen gradients are established remains controversial. In the case of Dpp two mechanisms have been postulated, namely extracellular diffusion and receptor-mediated transcytosis. In the first model Dpp is suggested to move by diffusion through the extracellular matrix of a tissue, whereas in the latter model Dpp is transported through the cells by receptor-mediated uptake and re-secretion. In this work we combined novel genetic tools with mathematical modeling to discriminate between the two models. Our results suggest that the Dpp gradient forms following the extracellular diffusion mechanism. Moreover, our data suggest that the majority of the extracellular Dpp is free and not bound to its receptor, a property likely to play a role for the long-range gradient formation.

Forming and interpreting gradients in the early Xenopus embryo

The amphibian embryo provides a powerful model system to study morphogen gradients because of the ease with which it is possible to manipulate the early embryo. In particular, it is possible to introduce exogenous sources of morphogen, to follow the progression of the signal, to monitor the cellular response to induction, and to up- or down-regulate molecules that are involved in all aspects of long-range signaling. In this article, I discuss the evidence that gradients exist in the early amphibian embryo, the way in which morphogens might traverse a field of cells, and the way in which different concentrations of morphogens might be interpreted to activate the expression of different genes.

Expression of Gpr177, a Wnt trafficking regulator, in mouse embryogenesis

Wls/Evi/Srt encoding a multipass transmembrane protein has been identified as a regulator for proper sorting and secretion of Wnt in flies. We have previously demonstrated that Gpr177 is the mouse ortholog required for axis determination. Gpr177 is a transcriptional target of Wnt that is activated to assist its subcellular distribution in a feedback regulatory loop. We, therefore, proposed that reciprocal regulation of Wnt and Gpr177 is essential for the Wnt-dependent developmental and pathogenic processes. Here, we examine the expression pattern of Gpr177 in mouse development. Gpr177 is expressed in a variety of tissues and cell types during organogenesis. Furthermore, Gpr177 is a glycoprotein primarily accumulating in the Golgi apparatus in signal-producing cells. The glycosylation of Gpr177 is necessary for proper transportation in the secretory pathway. Our findings suggest that the Gpr177-mediated regulation of Wnt is crucial for organogenesis in health and disease.

[Sorting, recycling and WNT signaling: Wntless and retromer functions]

Cell-cell signaling is essential for the development of multi-cellular organisms. Indeed, membrane traffic is required for the correct sorting and function of receptors and ligands. In the past decades, many genetic screens performed in C. elegans and Drosophila have been crucial to identify the role of intracellular traffic in signaling. In this review, we discuss recent work that led to the identification of Wntless, a sorting receptor for WNT, and of the retromer, a protein complex required for the recycling of Wntless from endosomes to the trans-Golgi network.

Endocytic regulation of TGF-beta signaling

Transforming growth factor-beta (TGF-beta) signaling is tightly regulated to ensure its proper physiological functions in different cells and tissues. Like other cell surface receptors, TGF-beta receptors are internalized into the cell, and this process plays an important regulatory role in TGF-beta signaling. It is well documented that TGF-beta receptors are endocytosed via clathrin-coated vesicles as TGF-beta endocytosis can be blocked by potassium depletion and the GTPase-deficient dynamin K44A mutant. TGF-beta receptors may also enter cells via cholesterol-rich membrane microdomain lipid rafts/caveolae and are found in caveolin-1-positive vesicles. Although receptor endocytosis is not essential for TGF-beta signaling, clathrin-mediated endocytosis has been shown to promote TGF-beta-induced Smad activation and transcriptional responses. Lipid rafts/caveolae are widely regarded as signaling centers for G protein-coupled receptors and tyrosine kinase receptors, but they are indicated to facilitate the degradation of TGF-beta receptors and therefore turnoff of TGF-beta signaling. This review summarizes current understanding of TGF-beta receptor endocytosis, the possible mechanisms underlying this process, and the role of endocytosis in modulation of TGF-beta signaling.

Geometric control of tissue morphogenesis

Morphogenesis is the dynamic and regulated change in tissue form that leads to creation of the body plan and development of mature organs. Research over the past several decades has uncovered a multitude of genetic factors required for morphogenesis in animals. The behaviors of individual cells within a developing tissue are determined by combining these genetic signals with information from the surrounding microenvironment. At any point in time, the local microenvironment is influenced by macroscale tissue geometry, which sculpts long range signals by affecting gradients of morphogens and mechanical stresses. The geometry of a tissue thus acts as both a template and instructive cue for further morphogenesis.

Itinerant exosomes: emerging roles in cell and tissue polarity

Cells use secreted signals (e.g. chemokines and growth factors) and sophisticated vehicles such as argosomes, cytonemes, tunneling nanotubes and exosomes to relay important information to other cells, often over large distances. Exosomes, 30-100-nm intraluminal vesicles of multivesicular bodies (MVB) released upon exocytic fusion of the MVB with the plasma membrane, are increasingly recognized as a novel mode of cell-independent communication. Exosomes have been shown to function in antigen presentation and tumor metastasis, and in transmitting infectious agents. However, little is known about the biogenesis and function of exosomes in polarized cells. In this review, we discuss new evidence suggesting that exosomes participate in the transport of morphogens and RNA, and thus influence cell polarity and developmental patterning of tissues.

Theoretical and experimental approaches to understand morphogen gradients

Morphogen gradients, which specify different fates for cells in a direct concentration-dependent manner, are a highly influential framework in which pattern formation processes in developmental biology can be characterized. A common analysis approach is combining experimental and theoretical strategies, thereby fostering relevant data on the dynamics and transduction of gradients. The mechanisms of morphogen transport and conversion from graded information to binary responses are some of the topics on which these combined strategies have shed light. Herein, we review these data, emphasizing, on the one hand, how theoretical approaches have been helpful and, on the other hand, how these have been combined with experimental strategies. In addition, we discuss those cases in which gradient formation and gradient interpretation at the molecular and/or cellular level may influence each other within a mutual feedback loop. To understand this interplay and the features it yields, it becomes essential to take system-level approaches that combine experimental and theoretical strategies.

Apical and lateral cell protrusions interconnect epithelial cells in live Drosophila wing imaginal discs

Communication among cells by means of the exchange of signaling cues is important for tissue and organ development. Recent reports indicate that one way that signaling cues can be delivered is by movement along cellular protrusions interconnecting cells. Here, by using confocal laser scanning microscopy and three-dimensional rendering, we describe in Drosophila melanogaster wing imaginal discs lateral protrusions interconnecting cells of the columnar epithelium. Moreover, we identified protrusions of the apical surface of columnar cells that reached and apparently contacted cells of the overlying squamous epithelium. Both apical and lateral protrusions could be visualized by expression of Tkv-GFP, a green fluorescent protein (GFP) -tagged version of a receptor of the Dpp/BMP4 signaling molecule, and the endosome marker GFP-Rab5. Our results demonstrate a previously unexpected richness of cellular protrusions within wing imaginal discs and support the view that cellular protrusions may provide a means for exchanging signaling cues between cells.

Wingless secretion requires endosome-to-Golgi retrieval of Wntless/Evi/Sprinter by the retromer complex

The glycolipoproteins of the Wnt family raise interesting trafficking issues, especially with respect to spreading within tissues. Recently, the retromer complex has been suggested to participate in packaging Wnts into long-range transport vehicles. Our analysis of a Drosophila mutant in Vps35 show that, instead, the retromer complex is required for efficient progression of Wingless (a Drosophila Wnt) through the secretory pathway. Indeed expression of senseless, a short-range target gene, is lost in Vps35-deficient imaginal discs. In contrast, Vps35 is not required for Hedgehog secretion, suggesting specificity. Overexpression of Wntless, a transmembrane protein known to be specifically required for Wingless secretion overcomes the secretion block of Vps35-mutant cells. Furthermore, biochemical evidence confirms that Wntless engages with the retromer complex. We propose that Wntless accompanies Wingless to the plasma membrane where the two proteins dissociate. Following dissociation from Wingless, Wntless is internalized and returns to the Golgi apparatus in a retromer-dependent manner. Without the retromer-dependent recycling route, Wingless secretion is impaired and, as electron microscopy suggests, Wntless is diverted to a degradative compartment.

Modeling homeoprotein intercellular transfer unveils a parsimonious mechanism for gradient and boundary formation in early brain development

Morphogens are molecules inducing morphogenetic responses from cells and cell ensembles. The concept of morphogen is related to that of positional value, as the generation of morphological and physiological characteristics is function of position. Based on the observation that homeoproteins, a category of transcription factors with morphogenetic functions, traffic between abutting cells and, very often, regulate their own expression, we develop here a biophysical model of homeoprotein propagation and study the associated mathematical equations. This mode of cell signaling can generate domains of homeoprotein expression. We study both the transient and steady-state regimes and, in this latter regime, we obtain various morphogenetic gradients, depending on the value of some parameters, such as morphogen synthesis, degradation rates and efficiency of intercellular passage. The same equations, applied to pairs of homeoproteins with auto-activation and reciprocal inhibition properties, account for border formation. They also allow us to compute how specific perturbations can either be buffered or lead to modifications in the position of borders between adjacent areas. The model developed here, based on experimental data, and avoids theoretical obstacles associated with pluricellularity. It extends the idea that Bicoid homeoprotein is a morphogen in the fly embryo syncitium to most homeoproteins and to pluricellular systems. Because the position of borders between brain areas is of primary physiological importance, our model might lead to original views regarding epigenetic inter-individual variations and the origin of neurological and psychiatric diseases. In addition, it provides new hypotheses regarding the molecular basis of brain evolution.

Modeling the bicoid gradient: diffusion and reversible nuclear trapping of a stable protein

The Bicoid gradient in the Drosophila embryo provided the first example of a morphogen gradient studied at the molecular level. The exponential shape of the Bicoid gradient had always been interpreted within the framework of the localized production, diffusion, and degradation model. We propose an alternative mechanism, which assumes no Bicoid degradation. The medium where the Bicoid gradient is formed and interpreted is very dynamic. Most notably, the number of nuclei changes over three orders of magnitude from fertilization, when Bicoid synthesis is initiated, to nuclear cycle 14 when most of the measurements were taken. We demonstrate that a model based on Bicoid diffusion and nucleocytoplasmic shuttling in the presence of the growing number of nuclei can account for most of the properties of the Bicoid concentration profile. Consistent with experimental observations, the Bicoid gradient in our model is established before nuclei migrate to the periphery of the embryo and remains stable during subsequent nuclear divisions.

Morphogenetic gradients and the stability of boundaries between neighboring morphogenetic regions

Generating morphogenetic gradients during early development is a fundamental step of positional signaling, which ultimately results in patterning and cell specialization. Based on morphogens propagation from cells to cells, we have presented a biophysical model in Holcman et al. (in press), where gradients and boundaries between different morphogenetic regions can be generated. In that theory, morphogens are transcription factors which induce their own activation and at the same time propagate in a cell ensemble. We analyze here a variant version of the biophysical model proposed in Holcman et al. (in press), where now morphogens can form dimers. As a result, gradients are smoother and borders are much sharper. Because random perturbations of a gradient can affect the precise location of the boundary between two morphogenetic regions, we also analyze these fluctuations and in particular, we obtain an analytic expression for the variance of the boundary location as a function of the variance of the random perturbations. This formula can be used to study the noise intrinsic effect on the boundary position between morphogenetic regions, which can be at the origin of interindividual variations.

The Hedgehog gene family of the cnidarian, Nematostella vectensis, and implications for understanding metazoan Hedgehog pathway evolution

Hedgehog signaling is an important component of cell-cell communication during bilaterian development, and abnormal Hedgehog signaling contributes to disease and birth defects. Hedgehog genes are composed of a ligand ("hedge") domain and an autocatalytic intein ("hog") domain. Hedgehog (hh) ligands bind to a conserved set of receptors and activate downstream signal transduction pathways terminating with Gli/Ci transcription factors. We have identified five intein-containing genes in the anthozoan cnidarian Nematostella vectensis, two of which (NvHh1 and NvHh2) contain definitive hedgehog ligand domains, suggesting that to date, cnidarians are the earliest branching metazoan phylum to possess definitive Hh orthologs. Expression analysis of NvHh1 and NvHh2, the receptor NvPatched, and a downstream transcription factor NvGli (a Gli3/Ci ortholog) indicate that these genes may have conserved roles in planar and trans-epithelial signaling during gut and germline development, while the three remaining intein-containing genes (NvHint1,2,3) are expressed in a cell-type-specific manner in putative neural precursors. Metazoan intein-containing genes that lack a hh ligand domain have previously only been identified within nematodes. However, we have identified intein-containing genes from both Nematostella and in two newly annotated lophotrochozoan genomes. Phylogenetic analyses suggest that while nematode inteins may be derived from an ancestral true hedgehog gene, the newly identified cnidarian and lophotrochozoan inteins may be orthologous, suggesting that both true hedgehog and hint genes may have been present in the cnidarian-bilaterian ancestor. Genomic surveys of N. vectensis suggest that most of the components of both protostome and deuterostome Hh signaling pathways are present in anthozoans and that some appear to have been lost in ecdysozoan lineages. Cnidarians possess many bilaterian cell-cell signaling pathways (Wnt, TGFbeta, FGF, and Hh) that appear to act in concert to pattern tissues along the oral-aboral axis of the polyp. Cnidarians represent a diverse group of animals with a predominantly epithelial body plan, and perhaps selective pressures to pattern epithelia resulted in the ontogeny of the hedgehog pathway in the common ancestor of the Cnidaria and Bilateria.

Lipoprotein-heparan sulfate interactions in the Hh pathway

The Drosophila lipoprotein particle, Lipophorin, bears lipid-linked morphogens on its surface and is required for long-range signaling activity of Wingless and Hedgehog. Heparan sulfate proteoglycans are also critical for trafficking and signaling of these morphogens. Here we show that Lipophorin interacts with the heparan sulfate moieties of the glypicans Dally and Dally-like. Membrane-associated glypicans can recruit Lipophorin to disc tissue, and remain associated with these particles after they are released from the membrane by cleavage of their gpi anchors. The released form of Dally colocalizes with Patched, Hedgehog, and Lipophorin in endosomes and increases Hedgehog signaling efficiency without affecting its distribution. These data suggest that heparan sulfate proteoglycans may influence lipid-linked morphogen signaling, at least in part, by binding to Lipophorin. They further suggest that the complement of proteins present on lipoprotein particles can regulate the activity of morphogens.

Diffusion in multilayer media: transient behavior of the lateral diffusion coefficient

A general formalism for treating lateral diffusion in a multilayer medium is developed. The formalism is based on the relation between the lateral diffusion and the distribution of the cumulative residence time, which the diffusing particle spends in different layers. We exploit this fact to derive general expressions which give the global and local time-dependent diffusion coefficients in terms of the average cumulative times spent by the particle in different layers and the probabilities of finding the particle in different layers, respectively. These expressions are used to generalize two recently obtained results: (a) A solution for the short-time behavior of the lateral diffusion coefficient in two layers separated by a permeable membrane obtained by a perturbation theory is extended to the entire range of time. (b) A solution for the time-dependent diffusion coefficient of a ligand, which repeatedly dissociates and rebinds to sites on a planar surface, obtained under the assumption that the medium above the surface is infinite, is generalized to allow for the medium layer of finite thickness. For the latter problem we derive an expression for the Fourier-Laplace transform of the propagator in terms of the double Laplace transform of the probability density of the cumulative residence time spent by the ligand in the medium layer.

Helping Wingless take flight: how WNT proteins are secreted

How functional WNT proteins are made and how their secretion is regulated is becoming a focal point for the WNT-signalling field. Recently, lipoprotein particles, WNT lipid modifications, the conserved transmembrane protein Wntless (WLS; also known as EVI and SRT) and the retromer complex have been implicated in WNT secretion. Our aim is to synthesize ideas from these new findings for the mechanisms that underlie WNT secretion.

The morphogenetic code and colon cancer development

The initiating genetic lesion in sporadically occurring cancers is impossible to identify. The existence of rare inherited cancer syndromes has helped to uncover some of the mutations that can initiate tumorigenesis. Most of these initiating lesions affect genes belonging to morphogenetic signaling pathways. We review the evidence that the cellular fate of individual epithelial cells in the adult is nonautonomous and depends on extrinsic information, just like cells in a developing embryo. Cancer stem cells need to disrupt these extrinsic restraints to gain an autonomous clonal proliferative advantage over neighboring stem cells.

Endocytosis, endosome trafficking, and the regulation of Drosophila development

Endocytosis and endosome trafficking regulate cell signaling in unexpected ways. Here we review the contribution that Drosophila research has made to this exciting field. In addition to attenuating signaling, endocytosis shapes morphogen gradients, activates ligands, and regulates spatially receptor activation within a single cell. Moreover, some receptors signal from within endosomes, and the ability of a specific type of endosome to form controls the ability of cells to signal. Experiments in Drosophila reveal that through regulation of a variety of cell signaling pathways, endocytosis controls cell patterning and cell fate.

Cell lineage transport: a mechanism for molecular gradient formation

Gradient formation is a fundamental patterning mechanism during embryo development, commonly related to secreted proteins that move along an existing field of cells. Here, we mathematically address the feasibility of gradients of mRNAs and non-secreted proteins. We show that these gradients can arise in growing tissues whereby cells dilute and transport their molecular content as they divide and grow, a mechanism we termed ‘cell lineage transport.' We provide an experimental test by unveiling a distal-to-proximal gradient of Hoxd13 in the vertebrate developing limb bud driven by cell lineage transport, corroborating our model. Our study indicates that gradients of non-secreted molecules exhibit a power-law profile and can arise for a wide range of biologically relevant parameter values. Dilution and nonlinear growth confer robustness to the spatial gradient under changes in the cell cycle period, but at the expense of sensitivity in the timing of gradient formation. We expect that gradient formation driven by cell lineage transport will provide future insights into understanding the coordination between growth and patterning during embryonic development.

Fab1 phosphatidylinositol 3-phosphate 5-kinase controls trafficking but not silencing of endocytosed receptors

The trafficking of endocytosed receptors through phosphatidylinositol 3-phosphate [PtdIns(3)P]-containing endosomes is thought to attenuate their signaling. Here, we show that the PtdIns(3)P 5-kinase Fab1/PIKfyve controls trafficking but not silencing of endocytosed receptors. Drosophila fab1 mutants contain undetectable phosphatidylinositol 3,5-bisphosphate levels, show profound increases in cell and organ size, and die at the pupal stage. Mutant larvae contain highly enlarged multivesicular bodies and late endosomes that are inefficiently acidified. Clones of fab1 mutant cells accumulate Wingless and Notch, similarly to cells lacking Hrs, Vps25, and Tsg101, components of the endosomal sorting machinery for ubiquitinated membrane proteins. However, whereas hrs, vps25, and tsg101 mutant cell clones accumulate ubiquitinated cargo, this is not the case with fab1 mutants. Even though endocytic receptor trafficking is impaired in fab1 mutants, Notch, Wingless, and Dpp signaling is unaffected. We conclude that Fab1, despite its importance for endosomal functions, is not required for receptor silencing. This is consistent with the possibility that Fab1 functions at a late stage in endocytic receptor trafficking, at a point when signal termination has occurred.

Quantitative Models of Developmental Pattern Formation

Pattern formation in developing organisms can be regulated at a variety of levels, from gene sequence to anatomy. At this level of complexity, mechanistic models of development become essential for integrating data, guiding future experiments, and predicting the effects of genetic and physical perturbations. However, the formulation and analysis of quantitative models of development are limited by high levels of uncertainty in experimental measurements, a large number of both known and unknown system components, and the multiscale nature of development. At the same time, an expanding arsenal of experimental tools can constrain models and directly test their predictions, making the modeling efforts not only necessary, but feasible. Using a number of problems in fruit fly development, we discuss how models can be used to test the feasibility of proposed patterning mechanisms and characterize their systems-level properties.

Epithelial trafficking of Sonic hedgehog by megalin

We present here evidence of in vivo epithelial endocytosis and trafficking of non-lipid-modified Sonic hedgehog (ShhN) when infused into rat efferent ducts via microinjection. Initially, exogenous ShhN is detected in endocytic vesicles and early endosomes located near the apical plasma membrane of non-ciliated cells. Within 30-60 min following infusion, ShhN can be detected in lysosomes and at basolateral regions of non-ciliated cells. Basolaterally, ShhN was observed along the extracellular surfaces of interdigitated plasma membranes of adjacent cells and in the extracellular compartment underlying the efferent duct epithelium. Uptake and subcellular trafficking of infused ShhN by non-ciliated cells could be blocked by either anti-megalin IgG or the megalin antagonist, RAP. Ciliated cells, which do not express megalin, displayed little if any apical internalization of ShhN even though they were found to express Patched-1. However, ShhN was found in coated pits of lateral plasma membranes of ciliated cells as well as in underlying endocytic vesicles. We conclude that megalin-mediated endocytosis of ShhN can occur in megalin-expressing epithelia in vivo, and that the internalized ShhN can be targeted to the lysosome or transcytosed in the plane of the epithelium or across the epithelium. These findings highlight the multiple mechanisms by which megalin may influence Shh morphogen gradients in vivo.

Wnt signals can function as positional cues in establishing cell polarity

Wnt signaling plays important roles in cell polarization in diverse organisms, and loss of cell polarity is an early event in tumorigenesis caused by mutations in Wnt pathway genes. Despite this, the precise roles of Wnt proteins in cell polarization have remained elusive. In no organism has it been shown that the asymmetric position of a Wnt signal is essential to establishing a cell's polarity. Attempts to test this by ubiquitous expression of Wnt genes have suggested that Wnt signals might act only as permissive factors in cell polarization. Here we find, by using cell manipulations and ectopic gene expression in C. elegans, that the position from which Wnt signals are presented can determine the polarity of both embryonic and postembryonic cells. Furthermore, the position from which a Wnt signal is presented can determine the polarity of Frizzled receptor localization, suggesting that the polarizing effect of Wnt is likely to be direct. These results demonstrate that Wnt proteins can function as positional cues in establishing cell polarity.

The interpretation of morphogen gradients

Morphogens act as graded positional cues that control cell fate specification in many developing tissues. This concept, in which a signalling gradient regulates differential gene expression in a concentration-dependent manner, provides a basis for understanding many patterning processes. It also raises several mechanistic issues, such as how responding cells perceive and interpret the concentration-dependent information provided by a morphogen to generate precise patterns of gene expression and cell differentiation in developing tissues. Here, we review recent work on the molecular features of morphogen signalling that facilitate the interpretation of graded signals and attempt to identify some emerging common principles.

Establishment and maintenance of planar epithelial cell polarity by asymmetric cadherin bridges: A computer model

Animal scales, hairs, feathers, and cilia are oriented due to cell polarization in the epithelial plane. Genes involved have been identified, but the signal and mechanism remain unknown. In Drosophila wing polarization, the action of a gradient of Frizzled activity is widely assumed; and cell-cell signalling by cadherins such as Flamingo surely plays a major role. We present a computer model where reading the Frizzled gradient occurs through biased, feedback-reinforced formation of Flamingo-based asymmetric intercellular complexes. Through these complexes neighboring cells are able to compare their Frizzled activity levels. Our computations are highly noise-resistant and reproduce both wild-type and all known mutant wing phenotypes; other phenotypes are predicted. The model puts stringent limits on a Frizzled activation signal, which should exhibit unusual properties: (1) the extracellular Frizzled signalling gradient should be counterdirectional--decreasing from proximal (P) to distal (D), whereas during polarization, the intracellular Frizzled gradient builds up from P to D; (2) the external gradient should be relatively weak and short-lived, lest it prevent inversion of intracellular Frizzled. These features, largely independent of model details, may provide useful clues for future experimental efforts.

Wingless signaling in a large insect, the blowflyLucilia sericata: A beautiful example of evolutionary developmental biology

Blowflies are the primary facultative agent in causing myiasis of domestic sheep in the whole world and, at the same time, it is an important tool for forensic medicine. Surprisingly, and in contrast to its importance, almost no data regarding the embryology and molecular markers are known for this insect. In this report, we present a detailed description of the blowfly Lucilia sericata embryogenesis and of imaginal disc development. The embryogenesis of Lucilia strongly resembles that of Drosophila, despite their apparent size difference. Moreover, imaginal disc development appears to be equally well conserved. Through cloning, expression, and functional studies, we show that the Lucilia Wingless (Wg) protein is highly conserved between the two species. We further show that parasegments are established in Lucilia, however, engrailed expression shows a more dynamic expression pattern than expected in comparison to Drosophila. Over-expression of Lucilia Wingless in Drosophila shows wingless-like wing phenotypes, suggesting that Lucilia Wingless blocks the signalling activity of Drosophila Wingless. Upon injection of wg dsRNA, we observe a "lawn of denticle" phenotype, closely resembling that of Drosophila. Due to the large size of the insect, the distance over which Wingless exerts signalling activity is up to three times larger than in Drosophila, yet the consequences are very similar. Our data demonstrate long-range wingless signaling mechanisms adapted for patterning large domains of naked cuticle and suggest signaling properties of Lucilia Wingless that are distinct from those of Drosophila Wingless.

Mechanisms of morphogen movement

Morphogens are defined as signaling molecules that are produced locally, yet act directly at a distance to pattern the surrounding field of cells in a concentration-dependent manner. In recent years many laboratories have devoted their attention to how morphogens actually reach distant cells. Several models have been proposed, including diffusion in the extracellular space and planar transcytosis. A combination of genetic, developmental, and cell-biological approaches have been taken to tackle this issue. I will present the models and discuss the types of experiments that have been designed to test them. It stands out that most of the work has been carried out in Drosophila. Morphogens contribute to patterning of the vertebrate nervous system, and the same signaling molecules have recently been shown to play important, possibly instructive, roles in axon guidance. Little, if anything, is known about the movement of morphogens in the context of nervous system development. The long-standing tradition of biophysical studies on diffusion in the brain extracellular space, along with the sophisticated in vitro culture systems developed in neurobiology laboratories, may provide new tools and ideas to test these models in a new context.

Mechanisms of Hedgehog gradient formation and interpretation

Morphogens are molecules that spread from localized sites of production, specifying distinct cell outcomes at different concentrations. Members of the Hedgehog (Hh) family of signaling molecules act as morphogens in different developmental systems. If we are to understand how Hh elicits multiple responses in a temporally and spatially specific manner, the molecular mechanism of Hh gradient formation needs to be established. Moreover, understanding the mechanisms of Hh signaling is a central issue in biology, not only because of the role of Hh in morphogenesis, but also because of its involvement in a wide range of human diseases. Here, we review the mechanisms affecting the dynamics of Hh gradient formation, mostly in the context of Drosophila wing development, although parallel findings in vertebrate systems are also discussed.

Morphogen gradient formation in a complex environment: an anomalous diffusion model

Current models of morphogen-induced patterning assume that morphogens undergo normal, or Fickian, diffusion, although the validity of this assumption has never been examined. Here we argue that the interaction of morphogens with the complex extracellular surrounding may lead to anomalous diffusion. We present a phenomenological model that captures this interaction, and derive the properties of the morphogen profile under conditions of anomalous (non-Fickian) diffusion. In this context we consider the continuous time random walk formalism and extend its application to account for degradation of morphogen particles. We show that within the anomalous diffusion model, morphogen profiles are fundamentally distinct from the corresponding Fickian profiles. Differences were found in several key aspects, including the role of degradation in determining the profile, the rate by which it spreads in time and its long-term behavior. We analyze the effect of an abrupt change in the extracellular environment on the concentration profiles. Furthermore, we discuss the robustness of the morphogen distribution to fluctuations in morphogen production rate, and describe a feedback mechanism that can buffer such fluctuations. Our study also provides rigorous criteria to distinguish experimentally between Fickian and anomalous modes of morphogen transport.

Nodal stability determines signaling range

Secreted TGFbeta proteins of the Nodal family pattern the vertebrate body axes and induce mesoderm and endoderm . Nodal proteins can act as morphogens , but the mechanisms regulating their activity and signaling range are poorly understood. In particular, it has been unclear how inefficient processing or rapid turnover of the Nodal protein influences autocrine and paracrine signaling properties . Here, we evaluate the role of Nodal processing and stability in tissue culture and zebrafish embryos. Removal of the pro domain potentiates autocrine signaling but reduces Nodal stability and signaling range. Insertion of an N-glycosylation site present in several related TGFbeta proteins increases the stability of mature Nodal. The stabilized form of Nodal acts at a longer range than the wild-type form. These results suggest that increased proteolytic maturation of Nodal potentiates autocrine signaling, whereas increased Nodal stability extends paracrine signaling.

A screen for genes regulating the wingless gradient in Drosophila embryos

During the development of the Drosophila embryonic epidermis, the secreted Wingless protein initially spreads symmetrically from its source. At later stages, Wingless becomes asymmetrically distributed in a Hedgehog-dependent manner, to control the patterning of the embryonic epidermis. When Wingless is misexpressed in engrailed cells in hedgehog heterozygous mutant embryos, larvae show a dominant phenotype consisting of patches of naked cuticle in denticle belts. This dose-sensitive phenotype is a direct consequence of a change in Wg protein distribution. We used this phenotype to carry out a screen for identifying genes regulating Wingless distribution or transport in the embryonic epidermis. Using a third chromosome deficiency collection, we found several genomic regions that showed a dominant interaction. After using a secondary screen to test for mutants and smaller deficiencies, we identified three interacting genes: dally, notum, and brahma. We confirmed that dally, as well as its homolog dally-like, and notum affect Wingless distribution in the embryonic epidermis, directly or indirectly. Thus, our assay can be used effectively to screen for genes regulating Wingless distribution or transport.

Interpreting clone-mediated perturbations of morphogen profiles

Generating clones of mutated cells within a wild-type tissue is a powerful experimental paradigm for elucidating gene function. Recently, this approach was employed for identifying genes that shape morphogen profiles in the Drosophila wing-imaginal disc. Interpreting such experiments poses a theoretical challenge. We present a general framework that links specific features of the morphogen profile in the clone vicinity to three basic morphogen properties: diffusion, degradation, and binding to immobile elements. Our results provide rigorous criteria to examine existing data and can facilitate the design and interpretation of future clone experiments.

Incredible journey: how do developmental signals travel through tissue?

How developmental signaling proteins traverse tissue during animal development, through or around tightly packed cells, remains an incompletely resolved mystery. Signaling protein movement is regulated to create gradients, control amounts, impose barriers, or provide direction. Signaling can be controlled by the rate of signal production, modification, active transport, trapping along the path, or by the properties of the receptor apparatus. Signals may move by diffusion outside cells, attached to migrating cells, attached to carrier molecules, through cells by transcytosis, along cell extensions, or in released membrane packets. Recent findings about the movement of Hedgehog, Wingless (Wnt), and TGF-beta signaling proteins have helped to clarify the molecular mechanisms used to ensure that developmental signals carry only good news.

The interface between cell and developmental biology

Cell differentiation, morphology, migration, polarity, intercellular communication and adhesion are all cellular processes that control embryo morphogenesis and lie at the interface of cell and developmental biology. The interface between these two fields is best illustrated, however, in studies of axiation and cytoskeletal remodeling during development. Recent advances reveal novel mechanisms for axiation, including the role of RNA and protein degradation in regulating the timely expression of morphogenetic signals. Significant progress has also been made in identifying components of the cytoskeleton and the extracellular matrix that mediate embryonic cell migration and polarity. Cellular processes at the interface of cell and developmental biology are overseen by the Wnt signaling cascade that coordinates both axiation and cytoskeletal remodeling during development.

Engineering gene networks to emulate Drosophila embryonic pattern formation

Pattern formation is essential in the development of higher eukaryotes. For example, in the Drosophila embryo, maternal morphogen gradients establish gap gene expression domain patterning along the anterior-posterior axis, through linkage with an elaborate gene network. To understand the evolution and behaviour of such systems better, it is important to establish the minimal determinants required for patterning. We have therefore engineered artificial transcription-translation networks that generate simple patterns, crudely analogous to the Drosophila gap gene system. The Drosophila syncytium was modelled using DNA-coated paramagnetic beads fixed by magnets in an artificial chamber, forming a gene expression network. Transient expression domain patterns were generated using various levels of network connectivity. Generally, adding more transcription repression interactions increased the “sharpness” of the pattern while reducing overall expression levels. An accompanying computer model for our system allowed us to search for parameter sets compatible with patterning. While it is clear that the Drosophila embryo is far more complex than our simplified model, several features of interest emerge. For example, the model suggests that simple diffusion may be too rapid for Drosophila-scale patterning, implying that sublocalisation, or “trapping,” is required. Second, we find that for pattern formation to occur under the conditions of our in vitro reaction-diffusion system, the activator molecules must propagate faster than the inhibitors. Third, adding controlled protease degradation to the system stabilizes pattern formation over time. We have reconstituted transcriptional pattern formation from purified substances, including phage RNA polymerases, ribonucleotides, and an eukaryotic translation extract. We anticipate that the system described here will be generally applicable to the study of any biological network with a spatial component.

To understand how patterns are established during early development, these authors have created an artificial system to mimic aspects of the early Drosophila embryo

Functions of heparan sulfate proteoglycans in cell signaling during development

Heparan sulfate proteoglycans (HSPGs) are cell-surface and extracellular matrix macromolecules that are composed of a core protein decorated with covalently linked glycosaminoglycan (GAG) chains. In vitro studies have demonstrated the roles of these molecules in many cellular functions, and recent in vivo studies have begun to clarify their essential functions in development. In particular, HSPGs play crucial roles in regulating key developmental signaling pathways, such as the Wnt, Hedgehog, transforming growth factor-beta, and fibroblast growth factor pathways. This review highlights recent findings regarding the functions of HSPGs in these signaling pathways during development.