Morphogen gradient interpretation by a regulated trafficking step during ligand-receptor transduction

Borcs6 is required for endo-lysosomal degradation during early development

Early development and differentiation require precise control of cellular functions. Lysosomal degradation is a critical component of normal cellular homeostasis, allowing for degradation of signaling molecules, proteins, and other macromolecules for cellular remodeling and signaling. Little is known about the role of lysosomal function in mammalian embryos before gastrulation. Borcs6 is a protein involved in lysosomal trafficking as well as endo-lysosomal and autophagosome fusion. Here, we show that Borcs6 is necessary for efficient endo-lysosomal degradation in the early embryo. Although embryos lacking Borcs6 are developmentally comparable to control littermates at E5.5, they are characterized by large cells containing increased levels of late endosomes and abnormal nuclei. Furthermore, these embryos display a skewed ratio of extraembryonic and embryonic cell lineages, are delayed by E6.5, and do not undergo normal gastrulation. These results demonstrate the essential functions of lysosomal positioning and fusion with endosomes during early embryonic development and indicate that the early lethality of BORCS6 mutant embryos is primarily due to defects in the HOPS-related function of BORC rather than lysosomal positioning.

Dynamic Visualization of TGF-β/SMAD3 Transcriptional Responses in Single Living Cells

Simple Summary

How a single cytokine can induce a variety of cellular responses in the same cell or in different cells is a longstanding question. Transforming growth factor β (TGF-β) is a prototypical multifunctional cytokine of which biological responses are highly dependent on in a cellular context. TGF-β signals via intracellular SMAD transcription factors, and the duration and intensity of SMAD activation are key determinants for the responses that are elicited by TGF-β. To visualize the TGF-β signaling kinetics, we developed a dynamic TGF-β/SMAD3 transcriptional reporter using a quickly folded and highly unstable green florescent protein. We demonstrate the specificity and sensitivity of this reporter and its wide application to monitor dynamic TGF-β-induced responses in cells cultured on plastic dishes, and in living animals. This tool allows for the analysis of TGF-β signaling at a single living cell level, and allows for the discovery of dynamic TGF-β SMAD- induced transcriptional responses in multi-step biological processes.

Abstract

Transforming growth factor-β (TGF-β) signaling is tightly controlled in duration and intensity during embryonic development and in the adult to maintain tissue homeostasis. To visualize the TGF-β/SMAD3 signaling kinetics, we developed a dynamic TGF-β/SMAD3 transcriptional fluorescent reporter using multimerized SMAD3/4 binding elements driving the expression of a quickly folded and highly unstable GFP protein. We demonstrate the specificity and sensitivity of this reporter and its wide application to monitor dynamic TGF-β/SMAD3 transcriptional responses in both 2D and 3D systems in vitro, as well as in vivo, using live-cell and intravital imaging. Using this reporter in B16F10 cells, we observed single cell heterogeneity in response to TGF-β challenge, which can be categorized into early, late, and non-responders. Because of its broad application potential, this reporter allows for new discoveries into how TGF-β/SMAD3-dependent transcriptional dynamics are affected during multistep and reversible biological processes.

Insights into the present and future of cartilage regeneration and joint repair

Knee osteoarthritis is the most common joint disease. It causes pain and suffering for affected patients and is the source of major economic costs for healthcare systems. Despite ongoing research, there is a lack of knowledge regarding disease mechanisms, biomarkers, and possible cures. Current treatments do not fulfill patients' long-term needs, and it often requires invasive surgical procedures with subsequent long periods of rehabilitation. Researchers and companies worldwide are working to find a suitable cell source to engineer or regenerate a functional and healthy articular cartilage tissue to implant in the damaged area. Potential cell sources to accomplish this goal include embryonic stem cells, mesenchymal stem cells, or induced pluripotent stem cells. The differentiation of stem cells into different tissue types is complex, and a suitable concentration range of specific growth factors is vital. The cellular microenvironment during early embryonic development provides crucial information regarding concentrations of signaling molecules and morphogen gradients as these are essential inducers for tissue development. Thus, morphogen gradients implemented in developmental protocols aimed to engineer functional cartilage tissue can potentially generate cells comparable to those within native cartilage. In this review, we have summarized the problems with current treatments, potential cell sources for cell therapy, reviewed the progress of new treatments within the regenerative cartilage field, and highlighted the importance of cell quality, characterization assays, and chemically defined protocols.

Embryonic stem cells are devoid of macropinocytosis, a trafficking pathway for activin A in differentiated cells

Ligand-receptor complexes formed at the plasma membrane are internalised via various endocytic pathways that influence the ultimate signalling output by regulating the selection of interaction partners by the complex along the trafficking route. We report that, in differentiated cells, activin A-receptor complexes are internalised via clathrin-mediated endocytosis (CME) and macropinocytosis (MP), whereas in human embryonic stem cells (hESCs) internalisation occurs via CME. We further show that hESCs are devoid of MP, which becomes functional upon differentiation towards endothelial cells through mesoderm mediators. Our results reveal, for the first time, that MP is an internalisation route for activin A in differentiated cells, and that MP is not active in hESCs and is induced as cells differentiate.

The pattern of nodal morphogen signaling is shaped by co-receptor expression

Embryos must communicate instructions to their constituent cells over long distances. These instructions are often encoded in the concentration of signals called morphogens. In the textbook view, morphogen molecules diffuse from a localized source to form a concentration gradient, and target cells adopt fates by measuring the local morphogen concentration. However, natural patterning systems often incorporate numerous co-factors and extensive signaling feedback, suggesting that embryos require additional mechanisms to generate signaling patterns. Here, we examine the mechanisms of signaling pattern formation for the mesendoderm inducer Nodal during zebrafish embryogenesis. We find that Nodal signaling activity spans a normal range in the absence of signaling feedback and relay, suggesting that diffusion is sufficient for Nodal gradient formation. We further show that the range of endogenous Nodal ligands is set by the EGF-CFC co-receptor Oep: in the absence of Oep, Nodal activity spreads to form a nearly uniform distribution throughout the embryo. In turn, increasing Oep levels sensitizes cells to Nodal ligands. We recapitulate these experimental results with a computational model in which Oep regulates the diffusive spread of Nodal ligands by setting the rate of capture by target cells. This model predicts, and we confirm in vivo, the surprising observation that a failure to replenish Oep transforms the Nodal signaling gradient into a travelling wave. These results reveal that patterns of Nodal morphogen signaling are shaped by co-receptor-mediated restriction of ligand spread and sensitization of responding cells.

Differentiation of induced pluripotent stem cells into definitive endoderm on Activin A-functionalized gradient surfaces

Activin A plays a central role in the differentiation of stem cells into definitive endoderm, the first step in embryonic development and function development in many organ systems. The aims of this study were to induce controlled and fine-tuned cell differentiation using a gradient nanotechnology and compare this with a classic protocol and to investigate how induced pluripotent stem cells differentiated depending on the gradual increase of Activin A. The density difference was tested by attaching Activin A to a gold nanoparticle gradient for high-precision density continuity. Cells expressed the definitive endoderm markers SRY-box transcription factor 17 and transcription factor GATA-4 to different extents along the gradient, indicating a density-dependent cell response to Activin A. In both the gradient and the classic differentiation setups, the protein expression increased from days 1 to 5, but a significant increase already on day 3 was found only in the gradient-based setup. By utilizing the gradient technology to present the right amount of active biomolecules to cells in vitro, we were able to find an optimal setting for differentiation into definitive endoderm. The use of gradient surfaces for differentiation allows for improvements, such as efficiency and faster differentiation, compared with a classic protocol.

Optogenetic approaches to investigate spatiotemporal signaling during development

Embryogenesis is coordinated by signaling pathways that pattern the developing organism. Many aspects of this process are not fully understood, including how signaling molecules spread through embryonic tissues, how signaling amplitude and dynamics are decoded, and how multiple signaling pathways cooperate to pattern the body plan. Optogenetic approaches can be used to address these questions by providing precise experimental control over a variety of biological processes. Here, we review how these strategies have provided new insights into developmental signaling and discuss how they could contribute to future investigations.

Setting up for gastrulation in zebrafish

Soon after fertilization the zebrafish embryo generates the pool of cells that will give rise to the germline and the three somatic germ layers of the embryo (ectoderm, mesoderm and endoderm). As the basic body plan of the vertebrate embryo emerges, evolutionarily conserved developmental signaling pathways, including Bmp, Nodal, Wnt, and Fgf, direct the nearly totipotent cells of the early embryo to adopt gene expression profiles and patterns of cell behavior specific to their eventual fates. Several decades of molecular genetics research in zebrafish has yielded significant insight into the maternal and zygotic contributions and mechanisms that pattern this vertebrate embryo. This new understanding is the product of advances in genetic manipulations and imaging technologies that have allowed the field to probe the cellular, molecular and biophysical aspects underlying early patterning. The current state of the field indicates that patterning is governed by the integration of key signaling pathways and physical interactions between cells, rather than a patterning system in which distinct pathways are deployed to specify a particular cell fate. This chapter focuses on recent advances in our understanding of the genetic and molecular control of the events that impart cell identity and initiate the patterning of tissues that are prerequisites for or concurrent with movements of gastrulation.

The Dynamics of TGF-β Signaling Are Dictated by Receptor Trafficking via the ESCRT Machinery

Signal transduction pathways stimulated by secreted growth factors are tightly regulated at multiple levels between the cell surface and the nucleus. The trafficking of cell surface receptors is emerging as a key step for regulating appropriate cellular responses, with perturbations in this process contributing to human diseases, including cancer. For receptors recognizing ligands of the transforming growth factor β (TGF-β) family, little is known about how trafficking is regulated or how this shapes signaling dynamics. Here, using whole genome small interfering RNA (siRNA) screens, we have identified the ESCRT (endosomal sorting complex required for transport) machinery as a crucial determinant of signal duration. Downregulation of ESCRT components increases the outputs of TGF-β signaling and sensitizes cells to low doses of ligand in their microenvironment. This sensitization drives an epithelial-to-mesenchymal transition (EMT) in response to low doses of ligand, and we demonstrate a link between downregulation of the ESCRT machinery and cancer survival.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Nodal and BMP dispersal during early zebrafish development

The secreted TGF-β superfamily signals Nodal and BMP coordinate the patterning of vertebrate embryos. Nodal specifies endoderm and mesoderm during germ layer formation, and BMP specifies ventral fates and patterns the dorsal/ventral axis. Five major models have been proposed to explain how the correct distributions of Nodal and BMP are achieved within tissues to orchestrate embryogenesis: source/sink, transcriptional determination, relay, self-regulation, and shuttling. Here, we discuss recent experiments probing these signal dispersal models, focusing on early zebrafish development.

An Effective Feedback Loop between Cell-Cell Contact Duration and Morphogen Signaling Determines Cell Fate

Cell-cell contact formation constitutes an essential step in evolution, leading to the differentiation of specialized cell types. However, remarkably little is known about whether and how the interplay between contact formation and fate specification affects development. Here, we identify a positive feedback loop between cell-cell contact duration, morphogen signaling, and mesendoderm cell-fate specification during zebrafish gastrulation. We show that long-lasting cell-cell contacts enhance the competence of prechordal plate (ppl) progenitor cells to respond to Nodal signaling, required for ppl cell-fate specification. We further show that Nodal signaling promotes ppl cell-cell contact duration, generating a positive feedback loop between ppl cell-cell contact duration and cell-fate specification. Finally, by combining mathematical modeling and experimentation, we show that this feedback determines whether anterior axial mesendoderm cells become ppl or, instead, turn into endoderm. Thus, the interdependent activities of cell-cell signaling and contact formation control fate diversification within the developing embryo.

Morphogen interpretation: concentration, time, competence, and signaling dynamics

Tissue patterning during animal development is orchestrated by a handful of inductive signals. Most of these developmental cues act as morphogens, meaning they are locally produced secreted molecules that act at a distance to govern tissue patterning. The iterative use of the same signaling molecules in different developmental contexts demands that signal interpretation occurs in a highly context‐dependent manner. Hence the interpretation of signal depends on the specific competence of the receiving cells. Moreover, it has become clear that the differential interpretation of morphogens depends not only on the level of signaling but also the signaling dynamics, particularly the duration of signaling. In this review, we outline molecular mechanisms proposed in recent studies that explain how the response to morphogens is determined by differential competence, pathway intrinsic feedback, and the interpretation of signaling dynamics by gene regulatory networks. WIREs Dev Biol 2017, 6:e271. doi: 10.1002/wdev.271

For further resources related to this article, please visit the WIREs website.

Vertebrate Axial Patterning: From Egg to Asymmetry

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal "organizer." This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.

A Balance between Secreted Inhibitors and Edge Sensing Controls Gastruloid Self-Organization

The earliest aspects of human embryogenesis remain mysterious. To model patterning events in the human embryo, we used colonies of human embryonic stem cells (hESCs) grown on micropatterned substrate and differentiated with BMP4. These gastruloids recapitulate the embryonic arrangement of the mammalian germ layers and provide an assay to assess the structural and signaling mechanisms patterning the human gastrula. Structurally, high-density hESCs localize their receptors to transforming growth factor β at their lateral side in the center of the colony while maintaining apical localization of receptors at the edge. This relocalization insulates cells at the center from apically applied ligands while maintaining response to basally presented ones. In addition, BMP4 directly induces the expression of its own inhibitor, NOGGIN, generating a reaction-diffusion mechanism that underlies patterning. We develop a quantitative model that integrates edge sensing and inhibitors to predict human fate positioning in gastruloids and, potentially, the human embryo.

Rab11, a vesicular trafficking protein, affects endoreplication through Ras-mediated pathway in Drosophila melanogaster

Rab11, a small monomeric GTPase associated with recycling endosomes, is a key molecule in the regulation of vesicular trafficking and is involved in the development and differentiation of many Drosophila tissues through interaction with diverse signaling pathways. In this study, we report for the first time that Rab11 affects endoreplication through a Ras-mediated pathway. Suppression of Rab11 activity in salivary glands, an endoreplicating tissue, leads to reduction in size of salivary glands with cells having a small nucleus. Endoreplication-regulating proteins, CycE, E2f1 and Gem, are also down-regulated in Rab11 knocked-down salivary glands suggesting that Rab11 has a role in the process of endoreplication, possibly indirectly through other pathways that regulate cell cycle progression. Ras signaling plays an important role in cell cycle progression through G/S phase transition. Ectopic expression of activated Ras in salivary glands of Rab11 down-regulated individuals rescues the small-sized glands to intermediate size. Furthermore, we observed altered localization of Ras in Rab11 down-regulated salivary glands. It is likely that the low level of endoreplication in the Rab11 down-regulated condition is Ras-mediated.

Semi-adaptive response and noise attenuation in bone morphogenetic protein signalling

Temporal dynamics of morphogen-driven signalling events are critical for proper embryonic development. During development, cells translate extracellular bone morphogenetic protein (BMP) gradients, often subject to noise, into graded intracellular tail-phosphorylated SMAD (TP-SMAD) levels. Using modelling and experimental approaches, we found that BMPs induce TP-SMAD responses in neural precursor cells in a concentration-dependent manner, which are semi-adaptive within a specific intermediate range of BMP concentration. These semi-adaptive TP-SMAD responses involve an intrinsically slow deactivation of BMP receptors, which attenuates noise by prolonging SMAD deactivation time after BMP withdrawal, but increases response time. Interestingly, negative feedback on BMP receptors is also required for semi-adaptation, which benefits both noise attenuation and response time, and therefore balances the trade-off seen with slow BMP receptor deactivation. These results highlight the rich dynamics of SMAD regulation in response to graded BMP concentration, and elucidate general design principles for balancing noise attenuation and activation speed in signalling systems.

A Temporal Window for Signal Activation Dictates the Dimensions of a Nodal Signaling Domain

Morphogen signaling is critical for the growth and patterning of tissues in embryos and adults, but how morphogen signaling gradients are generated in tissues remains controversial. The morphogen Nodal was proposed to form a long-range signaling gradient via a reaction-diffusion system, on the basis of differential diffusion rates of Nodal and its antagonist Lefty. Here we use a specific zebrafish Nodal biosensor combined with immunofluorescence for phosphorylated Smad2 to demonstrate that endogenous Nodal is unlikely to diffuse over a long range. Instead, short-range Nodal signaling activation in a temporal window is sufficient to determine the dimensions of the Nodal signaling domain. The size of this temporal window is set by the differentially timed production of Nodal and Lefty, which arises mainly from repression of Lefty translation by the microRNA miR-430. Thus, temporal information is transformed into spatial information to define the dimensions of the Nodal signaling domain and, consequently, to specify mesendoderm.

Response to Nodal morphogen gradient is determined by the kinetics of target gene induction

Morphogen gradients expose cells to different signal concentrations and induce target genes with different ranges of expression. To determine how the Nodal morphogen gradient induces distinct gene expression patterns during zebrafish embryogenesis, we measured the activation dynamics of the signal transducer Smad2 and the expression kinetics of long- and short-range target genes. We found that threshold models based on ligand concentration are insufficient to predict the response of target genes. Instead, morphogen interpretation is shaped by the kinetics of target gene induction: the higher the rate of transcription and the earlier the onset of induction, the greater the spatial range of expression. Thus, the timing and magnitude of target gene expression can be used to modulate the range of expression and diversify the response to morphogen gradients.

DOI:http://dx.doi.org/10.7554/eLife.05042.001

How a cell can tell where it is in a developing embryo has fascinated scientists for decades. The pioneering computer scientist and mathematical biologist Alan Turing was the first person to coin the term ‘morphogen’ to describe a protein that provides information about locations in the body. A morphogen is released from a group of cells (called the ‘source’) and as it moves away its activity (called the ‘signal’) declines gradually. Cells sense this signal gradient and use it to detect their position with respect to the source. Nodal is an important morphogen and is required to establish the correct identity of cells in the embryo; for example, it helps determine which cells should become a brain or heart or gut cell and so on.

The zebrafish is a widely used model to study animal development, in part because its embryos are transparent; this allows cells and proteins to be easily observed under a microscope. When Nodal acts on cells, another protein called Smad2 becomes activated, moves into the cell's nucleus, and then binds to specific genes. This triggers the expression of these genes, which are first copied into mRNA molecules via a process known as transcription and are then translated into proteins. The protein products of these targeted genes control cell identity and movement.

Several models have been proposed to explain how different concentrations of Nodal switch on the expression of different target genes; that is to say, to explain how a cell interprets the Nodal gradient. Dubrulle et al. have now measured factors that underlie how this gradient is interpreted. Individual cells in zebrafish embryos were tracked under a microscope, and Smad2 activation and gene expression were assessed. Dubrulle et al. found that, in contradiction to previous models, the amount of Nodal present on its own was insufficient to predict the target gene response. Instead, their analysis suggests that the size of each target gene's response depends on its rate of transcription and how quickly it is first expressed in response to Nodal.

These findings of Dubrulle et al. suggest that timing and transcription rate are important in determining the appropriate response to Nodal. Further work will be now needed to find out whether similar mechanisms regulate other processes that rely on the activity of morphogens.

DOI:http://dx.doi.org/10.7554/eLife.05042.002

Measuring protein stability in living zebrafish embryos using fluorescence decay after photoconversion (FDAP)

Protein stability influences many aspects of biology, and measuring the clearance kinetics of proteins can provide important insights into biological systems. In FDAP experiments, the clearance of proteins within living organisms can be measured. A protein of interest is tagged with a photoconvertible fluorescent protein, expressed in vivo and photoconverted, and the decrease in the photoconverted signal over time is monitored. The data is then fitted with an appropriate clearance model to determine the protein half-life. Importantly, the clearance kinetics of protein populations in different compartments of the organism can be examined separately by applying compartmental masks. This approach has been used to determine the intra- and extracellular half-lives of secreted signaling proteins during zebrafish development. Here, we describe a protocol for FDAP experiments in zebrafish embryos. It should be possible to use FDAP to determine the clearance kinetics of any taggable protein in any optically accessible organism.

Rab5 and Rab4 regulate axon elongation in the Xenopus visual system

The elongation rate of axons is tightly regulated during development. Recycling of the plasma membrane is known to regulate axon extension; however, the specific molecules involved in recycling within the growth cone have not been fully characterized. Here, we investigated whether the small GTPases Rab4 and Rab5 involved in short-loop recycling regulate the extension of Xenopus retinal axons. We report that, in growth cones, Rab5 and Rab4 proteins localize to endosomes, which accumulate markers that are constitutively recycled. Fluorescence recovery after photo-bleaching experiments showed that Rab5 and Rab4 are recruited to endosomes in the growth cone, suggesting that they control recycling locally. Dynamic image analysis revealed that Rab4-positive carriers can bud off from Rab5 endosomes and move to the periphery of the growth cone, suggesting that both Rab5 and Rab4 contribute to recycling within the growth cone. Inhibition of Rab4 function with dominant-negative Rab4 or Rab4 morpholino and constitutive activation of Rab5 decreases the elongation of retinal axons in vitro and in vivo, but, unexpectedly, does not disrupt axon pathfinding. Thus, Rab5- and Rab4-mediated control of endosome trafficking appears to be crucial for axon growth. Collectively, our results suggest that recycling from Rab5-positive endosomes via Rab4 occurs within the growth cone and thereby supports axon elongation.

Endocytosis and signaling during development

The development of multicellular organisms relies on an intricate choreography of intercellular communication events that pattern the embryo and coordinate the formation of tissues and organs. It is therefore not surprising that developmental biology, especially using genetic model organisms, has contributed significantly to the discovery and functional dissection of the associated signal-transduction cascades. At the same time, biophysical, biochemical, and cell biological approaches have provided us with insights into the underlying cell biological machinery. Here we focus on how endocytic trafficking of signaling components (e.g., ligands or receptors) controls the generation, propagation, modulation, reception, and interpretation of developmental signals. A comprehensive enumeration of the links between endocytosis and signal transduction would exceed the limits of this review. We will instead use examples from different developmental pathways to conceptually illustrate the various functions provided by endocytic processes during key steps of intercellular signaling.

A BMP-FGF morphogen toggle switch drives the ultrasensitive expression of multiple genes in the developing forebrain

Borders are important as they demarcate developing tissue into distinct functional units. A key challenge is the discovery of mechanisms that can convert morphogen gradients into tissue borders. While mechanisms that produce ultrasensitive cellular responses provide a solution, how extracellular morphogens drive such mechanisms remains poorly understood. Here, we show how Bone Morphogenetic Protein (BMP) and Fibroblast Growth Factor (FGF) pathways interact to generate ultrasensitivity and borders in the dorsal telencephalon. BMP and FGF signaling manipulations in explants produced border defects suggestive of cross inhibition within single cells, which was confirmed in dissociated cultures. Using mathematical modeling, we designed experiments that ruled out alternative cross inhibition mechanisms and identified a cross-inhibitory positive feedback (CIPF) mechanism, or "toggle switch", which acts upstream of transcriptional targets in dorsal telencephalic cells. CIPF explained several cellular phenomena important for border formation such as threshold tuning, ultrasensitivity, and hysteresis. CIPF explicitly links graded morphogen signaling in the telencephalon to switch-like cellular responses and has the ability to form multiple borders and scale pattern to size. These benefits may apply to other developmental systems.

Positive and negative regulation of developmental signaling by the endocytic pathway

Multicellular organisms acquire complex architecture through highly regulated developmental processes in which cells are programmed to respond to a specific set of extracellular signals produced by themselves and others. Modulation of sensitivity or duration of response is controlled by a variety of intracellular mechanisms. The endoocytic pathway performs essential regulatory roles both for the activation as well as the inactivation of signal transduction. Early stage of endocytic pathway is required for the recruitment of cytosolic mediators for signal amplification of signaling, whereas signal termination by late endosomes/lysosomes is important for spatiotemporal regulation. Herein, we summarize recent studies showing that dysfunction in endocytic pathways causes patterning defects in early embryogenesis in mammals.

Wnt regulation of planar cell polarity (PCP)

Planar cell polarity (PCP), a process controlling coordinated, uniformly polarized cellular behaviors in a field of cells, has been identified to be critically required for many fundamental developmental processes. However, a global directional cue that establishes PCP in a three-dimensional tissue or organ with respect to the body axes remains elusive. In vertebrate, while Wnt-secreted signaling molecules have been implicated in regulating PCP in a β-catenin-independent manner, whether they function permissively or act as a global cue to convey directional information is not clearly defined. In addition, the underlying molecular mechanism by which Wnt signal is transduced to core PCP proteins is largely unknown. In this chapter, I review the roles of Wnt signaling in regulating PCP during vertebrate development and update our knowledge of its regulatory mechanism.

Temporal control of BMP signalling determines neuronal subtype identity in the dorsal neural tube

The conventional explanation for how a morphogen patterns a tissue holds that cells interpret different concentrations of an extrinsic ligand by producing corresponding levels of intracellular signalling activity, which in turn regulate differential gene expression. However, this view has been challenged, raising the possibility that distinct mechanisms are used to interpret different morphogens. Here, we investigate graded BMP signalling in the vertebrate neural tube. We show that defined exposure times to Bmp4 generate distinct levels of signalling and induce specific dorsal identities. Moreover, we provide evidence that a dynamic gradient of BMP activity confers progressively more dorsal neural identities in vivo. These results highlight a strategy for morphogen interpretation in which the tight temporal control of signalling is important for the spatial pattern of cellular differentiation.

Active Wnt proteins are secreted on exosomes

Wnt signalling has important roles during development and in many diseases. As morphogens, hydrophobic Wnt proteins exert their function over a distance to induce patterning and cell differentiation decisions. Recent studies have identified several factors that are required for the secretion of Wnt proteins; however, how Wnts travel in the extracellular space remains a largely unresolved question. Here we show that Wnts are secreted on exosomes both during Drosophila development and in human cells. We demonstrate that exosomes carry Wnts on their surface to induce Wnt signalling activity in target cells. Together with the cargo receptor Evi/WIs, Wnts are transported through endosomal compartments onto exosomes, a process that requires the R-SNARE Ykt6. Our study demonstrates an evolutionarily conserved functional role of extracellular vesicular transport of Wnt proteins.

Reprint of: Betaglycan: a multifunctional accessory

Betaglycan is a co-receptor for the TGFβ superfamily, particularly important in establishing the potency of its ligands on their target cells. In recent years, new insights have been gained into the structure and function of betaglycan, expanding its role from that of a simple co-receptor to include additional ligand-dependent and ligand-independent roles. This review focuses on recent advances in the betaglycan field, with a particular emphasis on its newly discovered actions in mediating the trafficking of TGFβ superfamily receptors and as a determinant of the functional output of TGFβ superfamily signalling. In addition, this review encompasses a discussion of the emerging roles of the betaglycan/inhibin pathway in reproductive cancers and disease.

Distinct and separable activities of the endocytic clathrin-coat components Fcho1/2 and AP-2 in developmental patterning

Clathrin-mediated endocytosis occurs at multiple independent import sites on the plasma membrane, but how these positions are selected and how different cargo is simultaneously recognized is obscure. FCHO1 and FCHO2 are early-arriving proteins at surface clathrin assemblies and are speculated to act as compulsory coat nucleators, preceding the core clathrin adaptor AP-2. Here, we show the μ-homology domain (μHD) of FCHO1/2 represents a novel endocytic interaction hub. Translational silencing of fcho1 in zebrafish embryos causes strong dorsoventral patterning defects analogous to Bmp signal failure. The Fcho1 μHD interacts with the Bmp receptor Alk8, uncovering a new endocytic component that positively modulates Bmp signal transmission. Still, the fcho1 morphant phenotype is distinct from severe embryonic defects apparent when AP-2 is depleted. Our data thus contradict the primacy of FCHO1/2 in coat initiation.

Morphogen gradients in development: from form to function

Morphogens are substances that establish a graded distribution and elicit distinct cellular responses in a dose-dependent manner. They function to provide individual cells within a field with positional information, which is interpreted to give rise to spatial patterns. Morphogens can consist of intracellular factors that set up a concentration gradient by diffusion in the cytoplasm. More commonly, morphogens comprise secreted proteins that form an extracellular gradient across a field of cells. Experimental studies and computational analyses have provided support for a number of diverse strategies by which extracellular morphogen gradients are formed. These include free diffusion in the extracellular space, restricted diffusion aided by interactions with heparan sulfate proteoglycans, transport on lipid-containing carriers or transport aided by soluble binding partners. More specialized modes of transport have also been postulated such as transcytosis, in which repeated rounds of secretion, endocytosis, and intracellular trafficking move morphogens through cells rather than around them, or cytonemes, which consist of filopodial extensions from signal-receiving cells that are hypothesized to reach out to morphogen-sending cells. Once the gradient has formed, cells must distinguish small differences in morphogen concentration and store this information even after the gradient has dissipated. This is often achieved by translating ligand concentration into a proportional increase in numbers of activated cell surface receptors that are internalized and continue to signal from endosomal compartments. Ultimately, this leads to activation of one or a few transcription factors that transduce this information into qualitatively distinct gene responses inside the nucleus.

miRNAs and morphogen gradients

Morphogens induce biological diversity by operating in a dose-dependent manner. Here we review recent evidences indicating that microRNAs (miRNAs) are ideally suited to serve the morphogen cause. miRNAs regulate the establishment of morphogen gradients, including TGFβ, Wnt and other growth factors by acting on their secretion, distribution and clearance. miRNA are also critical in receiving cells, establishing context-dependency and threshold responses. Moreover, miRNAs contributes to gene networks that transform the graded activity of a morphogen into robust cell fate decisions. Finally, we discuss in the perspective section the implication of the new ceRNA hypothesis for morphogen biology.

Spatiotemporal mechanisms of morphogen gradient interpretation

Few mechanistic ideas from the pre-molecular era of biology have had as enduring an impact as the morphogen concept. In the classical view, cells in developing embryos obtain positional information by measuring morphogen concentrations and comparing them with fixed concentration thresholds; as a result, graded morphogen distributions map into discrete spatial arrangements of gene expression. Recent studies on Hedgehog and other morphogens suggest that establishing patterns of gene expression may be less a function of absolute morphogen concentrations, than of the dynamics of signal transduction, gene expression, and gradient formation. The data point away from any universal model of morphogen interpretation and suggest that organisms use multiple mechanisms for reading out developmental signals in order to accomplish specific patterning goals.

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.

Betaglycan: a multifunctional accessory

Betaglycan is a co-receptor for the TGFβ superfamily, particularly important in establishing the potency of its ligands on their target cells. In recent years, new insights have been gained into the structure and function of betaglycan, expanding its role from that of a simple co-receptor to include additional ligand-dependent and ligand-independent roles. This review focuses on recent advances in the betaglycan field, with a particular emphasis on its newly discovered actions in mediating the trafficking of TGFβ superfamily receptors and as a determinant of the functional output of TGFβ superfamily signalling. In addition, this review encompasses a discussion of the emerging roles of the betaglycan/inhibin pathway in reproductive cancers and disease.

New roles for lysosomal trafficking in morphogen gradient sensing

The way in which cells recognize their position in a gradient of morphogen controls differentiation during embryogenesis. New findings indicate that the rate at which internalized morphogen receptors are trafficked to lysosomes is key to the accurate and precise sensing of morphogen gradients and the appropriate initiation of differentiation programs during development.

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.

Nodal morphogens

Nodal signals belong to the TGF-beta superfamily and are essential for the induction of mesoderm and endoderm and the determination of the left-right axis. Nodal signals can act as morphogens-they have concentration-dependent effects and can act at a distance from their source of production. Nodal and its feedback inhibitor Lefty form an activator/inhibitor pair that behaves similarly to postulated reaction-diffusion models of tissue patterning. Nodal morphogen activity is also regulated by microRNAs, convertases, TGF-beta signals, coreceptors, and trafficking factors. This article describes how Nodal morphogens pattern embryonic fields and discusses how Nodal morphogen signaling is modulated.

Endocytosis is required for efficient apical constriction during Xenopus gastrulation

Coordinated apical constriction (AC) in epithelial sheets drives tissue invagination [1, 2] and is required for diverse morphogenetic movements such as gastrulation [3], neurulation [4, 5], and organogenesis [6]. We showed previously that actomyosin contractility drives AC in Xenopus laevis bottle cells [7]; however, it remained unclear whether it does so in concert with other processes. Here we report that endocytosis-driven membrane remodeling is required for efficient AC. We found endosomes exclusively in bottle cells in the early gastrula. Disrupting endocytosis with dominant-negative dynamin or rab5 perturbed AC, with a significant decrease in constriction rate late in the process, suggesting that endocytosis operates downstream of actomyosin contractility to remove excess membrane. Additionally, disrupting endocytosis during neurulation inhibits AC in hingepoint cells, resulting in neural tube closure defects. Thus, membrane remodeling during AC could be a general mechanism to achieve efficient invagination in embryos.

Rab5-mediated endocytosis of activin is not required for gene activation or long-range signalling in Xenopus

Morphogen gradients provide positional cues for cell fate specification and tissue patterning during embryonic development. One important aspect of morphogen function, the mechanism by which long-range signalling occurs, is still poorly understood. In Xenopus, members of the TGF-beta family such as the nodal-related proteins and activin act as morphogens to induce mesoderm and endoderm. In an effort to understand the mechanisms and dynamics of morphogen gradient formation, we have used fluorescently labelled activin to study ligand distribution and Smad2/Smad4 bimolecular fluorescence complementation (BiFC) to analyse, in a quantitative manner, the cellular response to induction. Our results indicate that labelled activin travels exclusively through the extracellular space and that its range is influenced by numbers of type II activin receptors on responding cells. Inhibition of endocytosis, by means of a dominant-negative form of Rab5, blocks internalisation of labelled activin, but does not affect the ability of cells to respond to activin and does not significantly influence signalling range. Together, our data indicate that long-range signalling in the early Xenopus embryo, in contrast to some other developmental systems, occurs through extracellular movement of ligand. Signalling range is not regulated by endocytosis, but is influenced by numbers of cognate receptors on the surfaces of responding cells.

Rock2 controls TGFbeta signaling and inhibits mesoderm induction in zebrafish embryos

The Rho-associated serine/threonine kinases Rock1 and Rock2 play important roles in cell contraction, adhesion, migration, proliferation and apoptosis. Here we report that Rock2 acts as a negative regulator of the TGFbeta signaling pathway. Mechanistically, Rock2 binds to and accelerates the lysosomal degradation of TGFbeta type I receptors internalized from the cell surface in mammalian cells. The inhibitory effect of Rock2 on TGFbeta signaling requires its kinase activity. In zebrafish embryos, injection of rock2a mRNA attenuates the expression of mesodermal markers during late blastulation and blocks the induction of mesoderm by ectopic Nodal signals. By contrast, overexpression of a dominant negative form of zebrafish rock2a, dnrock2a, has an opposite effect on mesoderm induction, suggesting that Rock2 proteins are endogenous inhibitors for mesoderm induction. Thus, our data have unraveled previously unidentified functions of Rock2, in controlling TGFbeta signaling as well as in regulating embryonic patterning.

Two highly related regulatory subunits of PP2A exert opposite effects on TGF-beta/Activin/Nodal signalling

We identify Balpha (PPP2R2A) and Bdelta (PPP2R2D), two highly related members of the B family of regulatory subunits of the protein phosphatase PP2A, as important modulators of TGF-beta/Activin/Nodal signalling that affect the pathway in opposite ways. Knockdown of Balpha in Xenopus embryos or mammalian tissue culture cells suppresses TGF-beta/Activin/Nodal-dependent responses, whereas knockdown of Bdelta enhances these responses. Moreover, in Drosophila, overexpression of Smad2 rescues a severe wing phenotype caused by overexpression of the single Drosophila PP2A B subunit Twins. We show that, in vertebrates, Balpha enhances TGF-beta/Activin/Nodal signalling by stabilising the basal levels of type I receptor, whereas Bdelta negatively modulates these pathways by restricting receptor activity. Thus, these highly related members of the same subfamily of PP2A regulatory subunits differentially regulate TGF-beta/Activin/Nodal signalling to elicit opposing biological outcomes.

Signaling from endosomes: location makes a difference

In all transmembrane receptor systems the kinetics of receptor trafficking upon ligand stimulation is maintained in a balance between degradative and recycling pathways in order to keep homeostasis and to strictly control receptor-mediated signaling. Endocytosis is commonly considered as an efficient mechanism of uptake and transport of membrane-associated signaling molecules leading to attenuation of ligand-induced responses. Accumulating evidence, however, shows that signaling from internalized receptors not only continues in endosomal compartments, but that there are also distinct signaling events that require endocytosis. Endocytic organelles form a dynamic network of subcellular compartments, which actively control the timing, amplitude, and specificity of signaling. In this review we provide examples in which signal transduction either requires an active endocytic machinery, or directly originates from various types of endosomes. Based on recent discoveries, we emphasize the close interdependence between signaling and endocytosis, and the physiological relevance of endocytic transport in health and disease.

Regulation of activin/nodal signaling by Rap2-directed receptor trafficking

We show that Rap2, a member of the Ras GTPase family, positively regulates Activin/Nodal signaling activity by controlling the trafficking of its receptors. In the absence of ligand activation, Rap2 directs internalized Activin/Nodal receptors into a recycling pathway, thereby preventing their degradation and maintaining their levels on the cell surface. Upon ligand activation, Rap2 no longer promotes receptor recycling but delays its turnover. In both cases, Rap2 contributes to upregulation of signaling activity by antagonizing Smad7. In addition, we found that the efficiency of Activin/Nodal receptor recycling is different between dorsal and ventral halves of Xenopus early embryo, which results from the asymmetric expression of Rap2 and Smad7. Consequently, they regulate cell responsiveness to ligands and the spatiotemporally dynamic activation of Smad2 along the dorsoventral axis of the embryo. Therefore, these findings suggest a molecular basis for the regulation of signaling activity and embryonic patterning by Activin/Nodal receptor trafficking.