Dynamic association with donor cell filopodia and lipid-modification are essential features of Wnt8a during patterning of the zebrafish neuroectoderm

Soluble Frizzled-related proteins promote exosome-mediated Wnt re-secretion

Wnt proteins are thought to be transported in several ways in the extracellular space. For instance, they are known to be carried by exosomes and by Wnt-carrier proteins, such as sFRP proteins. However, little is known about whether and/or how these two transport systems are related. Here, we show that adding sFRP1 or sFRP2, but not sFRP3 or sFRP4, to culture medium containing Wnt3a or Wnt5a increases re-secretion of exosome-loaded Wnt proteins from cells. This effect of sFRP2 is counteracted by heparinase, which removes sugar chains on heparan sulfate proteoglycans (HSPGs), but is independent of LRP5/6, Wnt co-receptors essential for Wnt signaling. Wnt3a and Wnt5a specifically dimerize with sFRP2 in culture supernatant. Furthermore, a Wnt3a mutant defective in heterodimerization with sFRP2 impairs the ability to increase exosome-mediated Wnt3a re-secretion. Based on these results, we propose that Wnt heterodimerization with its carrier protein, sFRP2, enhances Wnt accumulation at sugar chains on HSPGs on the cell surface, leading to increased endocytosis and exosome-mediated Wnt re-secretion. Our results suggest that the range of action of Wnt ligands is controlled by coordination of different transport systems.

Real-time monitoring of an endogenous Fgf8a gradient attests to its role as a morphogen during zebrafish gastrulation

Morphogen gradients impart positional information to cells in a homogenous tissue field. Fgf8a, a highly conserved growth factor, has been proposed to act as a morphogen during zebrafish gastrulation. However, technical limitations have so far prevented direct visualization of the endogenous Fgf8a gradient and confirmation of its morphogenic activity. Here, we monitor Fgf8a propagation in the developing neural plate using a CRISPR/Cas9-mediated EGFP knock-in at the endogenous fgf8a locus. By combining sensitive imaging with single-molecule fluorescence correlation spectroscopy, we demonstrate that Fgf8a, which is produced at the embryonic margin, propagates by diffusion through the extracellular space and forms a graded distribution towards the animal pole. Overlaying the Fgf8a gradient curve with expression profiles of its downstream targets determines the precise input-output relationship of Fgf8a-mediated patterning. Manipulation of the extracellular Fgf8a levels alters the signaling outcome, thus establishing Fgf8a as a bona fide morphogen during zebrafish gastrulation. Furthermore, by hindering Fgf8a diffusion, we demonstrate that extracellular diffusion of the protein from the source is crucial for it to achieve its morphogenic potential.

The logistics of Wnt production and delivery

Wnts are secreted proteins that control stem cell maintenance, cell fate decisions, and growth during development and adult homeostasis. Wnts carry a post-translational modification not seen in any other secreted protein: during biosynthesis, they are appended with a palmitoleoyl moiety that is required for signaling but also impairs solubility and hence diffusion in the extracellular space. In some contexts, Wnts act only in a juxtacrine manner but there are also instances of long range action. Several proteins and processes ensure that active Wnts reach the appropriate target cells. Some, like Porcupine, Wntless, and Notum are dedicated to Wnt function; we describe their activities in molecular detail. We also outline how the cell infrastructure (secretory, endocytic, and retromer pathways) contribute to the progression of Wnts from production to delivery. We then address how Wnts spread in the extracellular space and form a signaling gradient despite carrying a hydrophobic moiety. We highlight particularly the role of lipid-binding Wnt interactors and heparan sulfate proteoglycans. Finally, we briefly discuss how evolution might have led to the emergence of this unusual signaling pathway.

Extracellular Vesicles and Membrane Protrusions in Developmental Signaling

During embryonic development, cells communicate with each other to determine cell fate, guide migration, and shape morphogenesis. While the relevant secreted factors and their downstream target genes have been characterized extensively, how these signals travel between embryonic cells is still emerging. Evidence is accumulating that extracellular vesicles (EVs), which are well defined in cell culture and cancer, offer a crucial means of communication in embryos. Moreover, the release and/or reception of EVs is often facilitated by fine cellular protrusions, which have a history of study in development. However, due in part to the complexities of identifying fragile nanometer-scale extracellular structures within the three-dimensional embryonic environment, the nomenclature of developmental EVs and protrusions can be ambiguous, confounding progress. In this review, we provide a robust guide to categorizing these structures in order to enable comparisons between developmental systems and stages. Then, we discuss existing evidence supporting a role for EVs and fine cellular protrusions throughout development.

A lateral protrusion latticework connects neuroepithelial cells and is regulated during neurogenesis

Dynamic contacts between cells within the developing neuroepithelium are poorly understood but play important roles in cell and tissue morphology and cell signalling. Here, using live-cell imaging and electron microscopy we reveal multiple protrusive structures in neuroepithelial apical endfeet of the chick embryonic spinal cord, including sub-apical protrusions that extend laterally within the tissue, and observe similar structures in human neuroepithelium. We characterise the dynamics, shape and cytoskeleton of these lateral protrusions and distinguish them from cytonemes, filopodia and tunnelling nanotubes. We demonstrate that lateral protrusions form a latticework of membrane contacts between non-adjacent cells, depend on actin but not microtubule dynamics, and provide a lamellipodial-like platform for further extending fine actin-dependent filipodia. We find that lateral protrusions depend on the actin-binding protein WAVE1 (also known as WASF1): misexpression of mutant WAVE1 attenuated protrusion and generated a round-ended apical endfoot morphology. However, this did not alter apico-basal cell polarity or tissue integrity. During normal neuronal delamination, lateral protrusions were withdrawn, but precocious protrusion loss induced by mutant WAVE1 was insufficient to trigger neurogenesis. This study uncovers a new form of cell-cell contact within the developing neuroepithelium, regulation of which prefigures neuronal delamination. This article has an associated First Person interview with the first author of the paper.

Apelin signaling dependent endocardial protrusions promote cardiac trabeculation in zebrafish

During cardiac development, endocardial cells (EdCs) produce growth factors to promote myocardial morphogenesis and growth. In particular, EdCs produce neuregulin which is required for ventricular cardiomyocytes (CMs) to seed the multicellular ridges known as trabeculae. Defects in neuregulin signaling, or in endocardial sprouting toward CMs, cause hypotrabeculation. However, the mechanisms underlying endocardial sprouting remain largely unknown. Here, we first show by live imaging in zebrafish embryos that EdCs interact with CMs via dynamic membrane protrusions. After touching CMs, these protrusions remain in close contact with their target despite the vigorous cardiac contractions. Loss of the CM-derived peptide Apelin, or of the Apelin receptor, which is expressed in EdCs, leads to reduced endocardial sprouting and hypotrabeculation. Mechanistically, neuregulin signaling requires endocardial protrusions to induce extracellular signal-regulated kinase (Erk) activity in CMs and trigger their delamination. Altogether, these data show that Apelin signaling-dependent endocardial protrusions modulate CM behavior during trabeculation.

Vertebrate Wnt5a - At the crossroads of cellular signalling

Wnt signalling is an essential pathway in embryogenesis, differentiation, cell motility, development, and adult tissue homeostasis in vertebrates. The Wnt signalling network can activate several downstream pathways such as the β-catenin-dependent TCF/LEF transcription, the Wnt/planar cell polarity (PCP) pathway, and the Wnt/Calcium pathway. Wnt5a is a vertebrate Wnt ligand that is most often associated with the Wnt/PCP signalling pathway. Wnt5a/PCP signalling has a well-described role in embryogenesis via binding to a receptor complex of Frizzled and its co-receptors to initiate downstream activation of the c-Jun N-terminal kinase (JNK) signalling cascade and the Rho and Rac GTPases, Rho-Kinase (ROCK). This activation results in the cytoskeletal remodelling required for cell polarity, migration, and subsequently, tissue re-arrangement and organ formation. This review will focus on more recent work that has revealed new roles for Wnt5a ligands and consequently, an emerging broader function. This is partly due to our growing understanding of the crosstalk between the Wnt/PCP pathway with both the Wnt/β-catenin pathway and other signalling pathways, and in part due to the identification of novel atypical receptors for Wnt5a that demonstrate a far broader role for this ligand.

Beyond Neurons: Long Distance Communication in Development and Cancer

Cellular communication is important in all aspects of tissue and organism functioning, from the level of single cells, two discreet populations, and distant tissues of the body. Long distance communication networks integrate individual cells into tissues to maintain a complex organism during development, but when communication between cells goes awry, disease states such as cancer emerge. Herein we discuss the growing body of evidence suggesting that communication methods known to be employed by neurons, also exist in other cell types. We identify three major areas of long-distance communication: bioelectric signaling, tunneling nanotubes (TNTs), and macrophage modulation of networks, and draw comparisons about how these systems operate in the context of development and cancer. Bioelectric signaling occurs between cells through exchange of ions and tissue-level electric fields, leading to changes in biochemical gradients and molecular signaling pathways to control normal development and tumor growth and invasion in cancer. TNTs transport key morphogens and other cargo long distances, mediating electrical coupling, tissue patterning, and malignancy of cancer cells. Lastly macrophages maintain long distance signaling networks through trafficking of vesicles during development, providing communication relays and priming favorable microenvironments for cancer metastasis. By drawing comparisons between non-neural long distance signaling in the context of development and cancer we aim to encourage crosstalk between the two fields to cultivate new hypotheses and potential therapeutic strategies.

The Emerging Mechanisms of Wnt Secretion and Signaling in Development

Wnts are highly-conserved lipid-modified secreted proteins that activate multiple signaling pathways. These pathways regulate crucial processes during various stages of development and maintain tissue homeostasis in adults. One of the most fascinating aspects of Wnt protein is that despite being hydrophobic, they are known to travel several cell distances in the extracellular space. Research on Wnts in the past four decades has identified several factors and uncovered mechanisms regulating their expression, secretion, and mode of extracellular travel. More recently, analyses on the importance of Wnt protein gradients in the growth and patterning of developing tissues have recognized the complex interplay of signaling mechanisms that help in maintaining tissue homeostasis. This review aims to present an overview of the evidence for the various modes of Wnt protein secretion and signaling and discuss mechanisms providing precision and robustness to the developing tissues.

Regulation of Wnt Signaling Pathways at the Plasma Membrane and Their Misregulation in Cancer

Wnt signaling is one of the key signaling pathways that govern numerous physiological activities such as growth, differentiation and migration during development and homeostasis. As pathway misregulation has been extensively linked to pathological processes including malignant tumors, a thorough understanding of pathway regulation is essential for development of effective therapeutic approaches. A prominent feature of cancer cells is that they significantly differ from healthy cells with respect to their plasma membrane composition and lipid organization. Here, we review the key role of membrane composition and lipid order in activation of Wnt signaling pathway by tightly regulating formation and interactions of the Wnt-receptor complex. We also discuss in detail how plasma membrane components, in particular the ligands, (co)receptors and extracellular or membrane-bound modulators, of Wnt pathways are affected in lung, colorectal, liver and breast cancers that have been associated with abnormal activation of Wnt signaling. Wnt-receptor complex components and their modulators are frequently misexpressed in these cancers and this appears to correlate with metastasis and cancer progression. Thus, composition and organization of the plasma membrane can be exploited to develop new anticancer drugs that are targeted in a highly specific manner to the Wnt-receptor complex, rendering a more effective therapeutic outcome possible.

Protrusion-Mediated Signaling Regulates Patterning of the Developing Nervous System

During brain development, the tissue pattern and specification are the foundation of neuronal circuit formation. Contact-mediated lateral inhibition is well known to play an important role in determining cell fate decisions in the nervous system by either regulating tissue boundary formation or the classical salt-and-pepper pattern of differentiation that results from direct neighboring cell contacts. In many systems, however, such as the Drosophila notum, Drosophila wing, zebrafish pigmented cells, and zebrafish spinal cord, the differentiation pattern occurs at multiple-cell diameter distances. In this review, we discuss the evidence and characteristics of long-distance patterning mechanisms mediated by cellular protrusions. In the nervous system, cellular protrusions deliver the Notch ligand Delta at long range to prevent cells from differentiating in their vicinity. By temporal control of protrusive activity, this mechanism can pattern differentiation in both space and time.

Modeling of Wnt-mediated tissue patterning in vertebrate embryogenesis

During embryogenesis, morphogens form a concentration gradient in responsive tissue, which is then translated into a spatial cellular pattern. The mechanisms by which morphogens spread through a tissue to establish such a morphogenetic field remain elusive. Here, we investigate by mutually complementary simulations and in vivo experiments how Wnt morphogen transport by cytonemes differs from typically assumed diffusion-based transport for patterning of highly dynamic tissue such as the neural plate in zebrafish. Stochasticity strongly influences fate acquisition at the single cell level and results in fluctuating boundaries between pattern regions. Stable patterning can be achieved by sorting through concentration dependent cell migration and apoptosis, independent of the morphogen transport mechanism. We show that Wnt transport by cytonemes achieves distinct Wnt thresholds for the brain primordia earlier compared with diffusion-based transport. We conclude that a cytoneme-mediated morphogen transport together with directed cell sorting is a potentially favored mechanism to establish morphogen gradients in rapidly expanding developmental systems.

More Favorable Palmitic Acid Over Palmitoleic Acid Modification of Wnt3 Ensures Its Localization and Activity in Plasma Membrane Domains

While the lateral organization of plasma membrane components has been shown to control binding of Wnt ligands to their receptors preferentially in the ordered membrane domains, the role of posttranslational lipid modification of Wnt on this selective binding is unknown. Here, we identify that the canonical Wnt is presumably acylated by palmitic acid, a saturated 16-carbon fatty acid, at a conserved serine residue. Acylation of Wnt3 is dispensable for its secretion and binding to Fz8 while it is essential for Wnt3's proper binding and domain-like diffusion in the ordered membrane domains. We further unravel that non-palmitoylated Wnt3 is unable to activate Wnt/β-catenin signaling either in zebrafish embryos or in mammalian cells. Based on these results, we propose that the lipidation of canonical Wnt, presumably by a saturated fatty acid, determines its competence in interacting with the receptors in the appropriate domains of the plasma membrane, ultimately keeping the signaling activity under control.

Cripto-1 localizes to dynamic and shed filopodia associated with cellular migration in glioblastoma cells

Cripto-1 is a protein participating in tissue orientation during embryogenesis but has also been implicated in a wide variety of cancers, such as colon, lung and breast cancer. Cripto-1 plays a role in the regulation of different pathways, including TGF-β/Smad and Wnt/β-catenin, which are highly associated with cell migration both during embryonal development and cancer progression. Little is known about the detailed subcellular localization of cripto-1 and how it participates in the directional movement of cells. In this study, the subcellular localization of cripto-1 in glioblastoma cells was investigated in vitro with high-resolution microscopy techniques. Cripto-1 was found to be localized to dynamic and shed filopodia and transported between cells through tunneling nanotubes. Our results connect the refined subcellular localization of cripto-1 to its functions in cellular orientation and migration.

Mechanisms of intercellular Wnt transport

Wnt proteins are secreted glycoproteins that regulate multiple processes crucial to the development and tissue homeostasis of multicellular organisms, including tissue patterning, proliferation, cell fate specification, cell polarity and migration. To elicit these effects, Wnts act as autocrine as well as paracrine signalling molecules between Wnt-producing and Wnt-receiving cells. More than 40 years after the discovery of the Wg/Wnt pathway, it is still unclear how they are transported to fulfil their paracrine signalling functions. Several mechanisms have been proposed to mediate intercellular Wnt transport, including Wnt-binding proteins, lipoproteins, exosomes and cytonemes. In this Review, we describe the evidence for each proposed mechanism, and discuss how they may contribute to Wnt dispersal in tissue-specific and context-dependent manners, to regulate embryonic development precisely and maintain the internal steady state within a defined tissue.

A forebrain undivided: Unleashing model organisms to solve the mysteries of holoprosencephaly

Evolutionary conservation and experimental tractability have made animal model systems invaluable tools in our quest to understand human embryogenesis, both normal and abnormal. Standard genetic approaches, particularly useful in understanding monogenic diseases, are no longer sufficient as research attention shifts toward multifactorial outcomes. Here, we examine this progression through the lens of holoprosencephaly (HPE), a common human malformation involving incomplete forebrain division, and a classic example of an etiologically complex outcome. We relate the basic underpinning of HPE pathogenesis to critical cell-cell interactions and signaling molecules discovered through embryological and genetic approaches in multiple model organisms, and discuss the role of the mouse model in functional examination of HPE-linked genes. We then outline the most critical remaining gaps to understanding human HPE, including the conundrum of incomplete penetrance/expressivity and the role of gene-environment interactions. To tackle these challenges, we outline a strategy that leverages new and emerging technologies in multiple model systems to solve the puzzle of HPE.

Assembly of protein complexes restricts diffusion of Wnt3a proteins

Members of the Wnt protein family play roles in many aspects of embryogenesis and homeostasis. Despite their biological significance, characteristics of Wnt proteins still remain unclear, mainly due to their insolubility after the removal of serum. Here we examine Wnt proteins in serum-containing media by using analytical ultracentrifugation with a fluorescence detection system. This analysis reveals that Wnt3a assembles into high-molecular-weight complexes that become dissociable by interaction with the extracellular domain of the Frizzled8 receptor or secreted Wnt-binding protein sFRP2. Cross-linking and single-particle analyses of Wnt3a fractionated by gel filtration chromatography show the homo-trimer to be the smallest form of the assembled Wnt3a complexes. Fluorescence correlation spectroscopy and immunohistochemistry reveal that the assembly of Wnt3a complexes restricted their diffusion and signaling range in Xenopus laevis embryos. Thus, we propose that the Wnt diffusion range can be controlled by a balance between the assembly of Wnt complexes and their dissociation.

Ritsuko Takada et al. show that Wnt3a assembles into high molecular weight complexes that restrict the diffusion of Wnt within Xenopus embryos. These results suggest that Wnt diffusion in cells is controlled by a balance between higher order complex assembly and dissociation by Wnt-binding proteins.

Emerging role of contact-mediated cell communication in tissue development and diseases

Cells of multicellular organisms are in continuous conversation with the neighbouring cells. The sender cells signal the receiver cells to influence their behaviour in transport, metabolism, motility, division, and growth. How cells communicate with each other can be categorized by biochemical signalling processes, which can be characterised by the distance between the sender cell and the receiver cell. Existing classifications describe autocrine signals as those where the sender cell is identical to the receiver cell. Complementary to this scenario, paracrine signalling describes signalling between a sender cell and a different receiver cell. Finally, juxtacrine signalling describes the exchange of information between adjacent cells by direct cell contact, whereas endocrine signalling describes the exchange of information, e.g., by hormones between distant cells or even organs through the bloodstream. In the last two decades, however, an unexpected communication mechanism has been identified which uses cell protrusions to exchange chemical signals by direct contact over long distances. These signalling protrusions can deliver signals in both ways, from sender to receiver and vice versa. We are starting to understand the morphology and function of these signalling protrusions in many tissues and this accumulation of findings forces us to revise our view of contact-dependent cell communication. In this review, we will focus on the two main categories of signalling protrusions, cytonemes and tunnelling nanotubes. These signalling protrusions emerge as essential structural components of a vibrant communication network in the development and tissue homeostasis of any multicellular organism.

Release and spread of Wingless is required to pattern the proximo-distal axis of Drosophila renal tubules

Wingless/Wnts are signalling molecules, traditionally considered to pattern tissues as long-range morphogens. However, more recently the spread of Wingless was shown to be dispensable in diverse developmental contexts in Drosophila and vertebrates. Here we demonstrate that release and spread of Wingless is required to pattern the proximo-distal (P-D) axis of Drosophila Malpighian tubules. Wingless signalling, emanating from the midgut, directly activates odd skipped expression several cells distant in the proximal tubule. Replacing Wingless with a membrane-tethered version that is unable to diffuse from the Wingless producing cells results in aberrant patterning of the Malpighian tubule P-D axis and development of short, deformed ureters. This work directly demonstrates a patterning role for a released Wingless signal. As well as extending our understanding about the functional modes by which Wnts shape animal development, we anticipate this mechanism to be relevant to patterning epithelial tubes in other organs, such as the vertebrate kidney.

Specialized Intercellular Communications via Cytonemes and Nanotubes

In recent years, thin membrane protrusions such as cytonemes and tunneling nanotubes have emerged as a novel mechanism of intercellular communication. Protrusion-based cellular interactions allow for specific communication between participating cells and have a distinct spectrum of advantages compared to secretion- and diffusion-based intercellular communication. Identification of protrusion-based signaling in diverse systems suggests that this mechanism is a ubiquitous and prevailing means of communication employed by many cell types. Moreover, accumulating evidence indicates that protrusion-based intercellular communication is often involved in pathogenesis, including cancers and infections. Here we review our current understanding of protrusion-based intercellular communication.

Wnt/PCP controls spreading of Wnt/β-catenin signals by cytonemes in vertebrates

Signaling filopodia, termed cytonemes, are dynamic actin-based membrane structures that regulate the exchange of signaling molecules and their receptors within tissues. However, how cytoneme formation is regulated remains unclear. Here, we show that Wnt/planar cell polarity (PCP) autocrine signaling controls the emergence of cytonemes, and that cytonemes subsequently control paracrine Wnt/β-catenin signal activation. Upon binding of the Wnt family member Wnt8a, the receptor tyrosine kinase Ror2 becomes activated. Ror2/PCP signaling leads to the induction of cytonemes, which mediate the transport of Wnt8a to neighboring cells. In the Wnt-receiving cells, Wnt8a on cytonemes triggers Wnt/β-catenin-dependent gene transcription and proliferation. We show that cytoneme-based Wnt transport operates in diverse processes, including zebrafish development, murine intestinal crypt and human cancer organoids, demonstrating that Wnt transport by cytonemes and its control via the Ror2 pathway is highly conserved in vertebrates.

Communication helps the cells that make up tissues and organs to work together as a team. One way that cells share information with each other as tissues grow and develop is by exchanging signaling proteins. These interact with receptors on the surface of other cells; this causes the cell to change how it behaves.

The Wnt family of signaling proteins orchestrate organ development. Wnt proteins influence which types of cells develop, how fast they divide, and how and when they move. Relatively few cells, or small groups of cells, in developing tissues produce Wnt proteins, while larger groups nearby respond to the signals.

We do not fully understand how Wnt proteins travel between cells, but recent work revealed an unexpected mechanism – cells seem to hand-deliver their messages. Finger-like structures called cytonemes grow out of the cell membrane and carry Wnt proteins to their destination. If the cytonemes do not form properly the target cells do not behave correctly, which can lead to severe tissue malformation.

Mattes et al. have now investigated how cytonemes form using a combination of state-of-the-art genetic and high-resolution imaging techniques. In initial experiments involving zebrafish cells that were grown in the laboratory, Mattes et al. found that the Wnt proteins kick start their own transport; before they travel to their destination, they act on the cells that made them. A Wnt protein called Wnt8a activates the receptor Ror2 on the surface of the signal-producing cell. Ror2 then triggers signals inside the cell that begin the assembly of the cytonemes. The more Ror2 is activated, the more cytonemes the cell makes, and the more Wnt signals it can send out.

This mechanism operates in various tissues: Ror2 also controls the cytoneme transport process in living zebrafish embryos, the mouse intestine and human stomach tumors. This knowledge will help researchers to develop new ways to control Wnt signaling, which could help to produce new treatments for diseases ranging from cancers (for example in the stomach and bowel) to degenerative diseases such as Alzheimer’s disease.

Direct visualization of the Wntless-induced redistribution of WNT1 in developing chick embryos

Paracrine Wnt signals are critical regulators of cell proliferation, specification, and differentiation during embryogenesis. Consistent with the discovery that Wnt ligands are post-translationally modified with palmitoleate (a 16 carbon mono-unsaturated fatty acid), our studies show that the vast majority of bioavailable chick WNT1 (cWNT1) produced in stably transfected L cells is cell-associated. Thus, it seems unlikely that the WNT1 signal is propagated by diffusion alone. Unfortunately, the production and transport of vertebrate Wnt proteins has been exceedingly difficult to study as few antibodies are able to detect endogenous Wnt proteins and fixation is known to disrupt the architecture of cells and tissues. Furthermore, vertebrate Wnts have been extraordinarily refractory to tagging. To help overcome these obstacles, we have generated a number of tools that permit the detection of WNT1 in palmitoylation assays and the visualization of chick and zebrafish WNT1 in live cells and tissues. Consistent with previous studies in fixed cells, live imaging of cells and tissues with overexpressed cWNT1-moxGFP shows predominant localization of the protein to a reticulated network that is likely to be the endoplasmic reticulum. As PORCN and WLS are important upstream regulators of Wnt gradient formation, we also undertook the generation of mCherry-tagged variants of both proteins. While co-expression of PORCN-mCherry had no discernible effect on the localization of WNT1-moxGFP, co-expression of WLS-mCherry caused a marked redistribution of WNT1-moxGFP to the cell surface and cellular projections in cultured cells as well as in neural crest and surface ectoderm cells in developing chick embryos. Our studies further establish that the levels of WLS, and not PORCN, are rate limiting with respect to WNT1 trafficking.

From top to bottom: Cell polarity in Hedgehog and Wnt trafficking

Spatial organization of membrane domains within cells and cells within tissues is key to the development of organisms and the maintenance of adult tissue. Cell polarization is crucial for correct cell-cell signalling, which, in turn, promotes cell differentiation and tissue patterning. However, the mechanisms linking internal cell polarity to intercellular signalling are just beginning to be unravelled. The Hedgehog (Hh) and Wnt pathways are major directors of development and their malfunction can cause severe disorders like cancer. Here we discuss parallel advances into understanding the mechanism of Hedgehog and Wnt signal dissemination and reception. We hypothesize that cell polarization of the signal-sending and signal-receiving cells is crucial for proper signal spreading and activation of the pathway and, thus, fundamental for development of multicellular organisms.

The function of endocytosis in Wnt signaling

Wnt growth factors regulate one of the most important signaling networks during development, tissue homeostasis and disease. Despite the biological importance of Wnt signaling, the mechanism of endocytosis during this process is ill described. Wnt molecules can act as paracrine signals, which are secreted from the producing cells and transported through neighboring tissue to activate signaling in target cells. Endocytosis of the ligand is important at several stages of action: One central function of endocytic trafficking in the Wnt pathway occurs in the source cell. Furthermore, the β-catenin-dependent Wnt ligands require endocytosis for signal activation and to regulate gene transcription in the responding cells. Alternatively, Wnt/β-catenin-independent signaling regulates endocytosis of cell adherence plaques to control cell migration. In this comparative review, we elucidate these three fundamental interconnected functions, which together regulate cellular fate and cellular behavior. Based on established hypotheses and recent findings, we develop a revised picture for the complex function of endocytosis in the Wnt signaling network.

Single-Molecule Imaging of Wnt3A Protein Diffusion on Living Cell Membranes

Wnt proteins are secreted, hydrophobic, lipidated proteins found in all animals that play essential roles in development and disease. Lipid modification is thought to facilitate the interaction of the protein with its receptor, Frizzled, but may also regulate the transport of Wnt protein and its localization at the cell membrane. Here, by employing single-molecule fluorescence techniques, we show that Wnt proteins associate with and diffuse on the plasma membranes of living cells in the absence of any receptor binding. We find that labeled Wnt3A transiently and dynamically associates with the membranes of Drosophila Schneider 2 cells, diffuses with Brownian kinetics on flattened membranes and on cellular protrusions, and does not transfer between cells in close contact. In S2 receptor-plus (S2R+) cells, which express Frizzled receptors, membrane diffusion rate is reduced and membrane residency time is increased. These results provide direct evidence of Wnt3A interaction with living cell membranes, and represent, to our knowledge, a new system for investigating the dynamics of Wnt transport.

Roles of two types of heparan sulfate clusters in Wnt distribution and signaling in Xenopus

Wnt proteins direct embryonic patterning, but the regulatory basis of their distribution and signal reception remain unclear. Here, we show that endogenous Wnt8 protein is distributed in a graded manner in Xenopus embryo and accumulated on the cell surface in a punctate manner in association with “N-sulfo-rich heparan sulfate (HS),” not with “N-acetyl-rich HS”. These two types of HS are differentially clustered by attaching to different glypicans as core proteins. N-sulfo-rich HS is frequently internalized and associated with the signaling vesicle, known as the Frizzled/Wnt/LRP6 signalosome, in the presence of Wnt8. Conversely, N-acetyl-rich HS is rarely internalized and accumulates Frzb, a secreted Wnt antagonist. Upon interaction with Frzb, Wnt8 associates with N-acetyl-rich HS, suggesting that N-acetyl-rich HS supports Frzb-mediated antagonism by sequestering Wnt8 from N-sulfo-rich HS. Thus, these two types of HS clusters may constitute a cellular platform for the distribution and signaling of Wnt8.

Wnt proteins mediate embryonic development but how protein localization and patterning is regulated is unclear. Here, the authors show that distinct structures with different heparan sulfate modifications (‘N-sulfo-rich’ and ‘N-acetyl-rich’) regulate cellular localization and signal transduction of Wnt8 in Xenopus.

Concepts for a therapeutic prolongation of nephrogenesis in preterm and low-birth-weight babies must correspond to structural-functional properties in the nephrogenic zone

Numerous investigations are dealing with anlage of the mammalian kidney and primary development of nephrons. However, only few information is available about the last steps in kidney development leading at birth to a downregulation of morphogen activity in the nephrogenic zone and to a loss of stem cell niches aligned beyond the organ capsule. Surprisingly, these natural changes in the developmental program display similarities to processes occurring in the kidneys of preterm and low-birth-weight babies. Although those babies are born at a time with a principally intact nephrogenic zone and active niches, a high proportion of them suffers on impairment of nephrogenesis resulting in oligonephropathy, formation of atypical glomeruli, and immaturity of parenchyma. The setting points out that up to date not identified noxae in the nephrogenic zone hamper primary steps of parenchyma development. In this situation, a possible therapeutic aim is to prolong nephrogenesis by medications. However, actual data provide information that administration of drugs is problematic due to an unexpectedly complex microanatomy of the nephrogenic zone, in niches so far not considered textured extracellular matrix and peculiar contacts between mesenchymal cell projections and epithelial stem cells via tunneling nanotubes. Thus, it remains to be figured out whether disturbance of morphogen signaling altered synthesis of extracellular matrix, disturbed cell-to-cell contacts, or modified interstitial fluid impair nephrogenic activity. Due to most unanswered questions, search for eligible drugs prolonging nephrogenesis and their reliable administration is a special challenge for the future.

Effects of genetic variants of the bovine WNT8A gene on nine important growth traits in beef cattle

WNT-β-catenin-TCF pathway is involved in carcinogenesis and foetal development. As a member of the WNT gene family, Wnt8A encodes secreted signalling proteins and responds to many biological processes.However, similar research on the effects of genetic variations of Wnt8A gene on growth traits is lacking. Therefore, in this study, polymorphisms of Wnt8A were detected in 396 animals from Chinese Qinchuan cattle using DNA pool sequencing and PCR-RFLP methods. Four novel single-nucleotide polymorphisms (SNPs) of Wnt8A gene were identified, including three mutations in introns (g.T-445C, g.G244C and g.G910A) and one in exon (g.T4922C). Additionally, we examined the associations of four SNPs with growth traits. The results revealed that SNP2 (g.G244C) was significantly associated with shoulder height, hip height, body length, hip width, and body weight (P < 0.05). SNP3 (g.G910A) also displayed notable effects on hip width (P < 0.05). Meanwhile, the haplotype combination CC-GC-GA-CC was strongly associated with heavier, taller and longer animals (P < 0.05). These results show that the Wnt8A gene may be a potential candidate gene, and the SNPs could be used as molecular markers in early marker-assisted selection in beef cattle breeding programmes.

Wnt ligand presentation and reception: from the stem cell niche to tissue engineering

Stem cells reside in niches where spatially restricted signals maintain a delicate balance between stem cell self-renewal and differentiation. Wnt family proteins are particularly suited for this role as they are modified by lipids, which constrain and spatially regulate their signalling range. In recent years, Wnt/β-catenin signalling has been shown to be essential for the self-renewal of a variety of mammalian stem cells. In this review, we discuss Wnt-responsive stem cells in their niche, and mechanisms by which Wnt ligands are presented to responsive cells. We also highlight recent progress in molecular visualization that has allowed for the monitoring of Wnt signalling within the stem cell compartment and new approaches to recapitulate this niche signalling in vitro Indeed, new technologies that present Wnt in a localized manner and mimic the three-dimensional microenvironment of stem cells will advance our understanding of Wnt signalling in the stem cell niche. These advances will expand current horizons to exploit Wnt ligands in the rapidly evolving fields of tissue engineering and regenerative medicine.

Binding of canonical Wnt ligands to their receptor complexes occurs in ordered plasma membrane environments

While the cytosolic events of Wnt/β‐catenin signaling (canonical Wnt signaling) pathway have been widely studied, only little is known about the molecular mechanisms involved in Wnt binding to its receptors at the plasma membrane. Here, we reveal the influence of the immediate plasma membrane environment on the canonical Wnt–receptor interaction. While the receptors are distributed both in ordered and disordered environments, Wnt binding to its receptors selectively occurs in more ordered membrane environments which appear to cointernalize with the Wnt‐receptor complex. Moreover, Wnt/β‐catenin signaling is significantly reduced when the membrane order is disturbed by specific inhibitors of certain lipids that prefer to localize at the ordered environments. Similarly, a reduction in Wnt signaling activity is observed in Niemann–Pick Type C disease cells where trafficking of ordered membrane lipid components to the plasma membrane is genetically impaired. We thus conclude that ordered plasma membrane environments are essential for binding of canonical Wnts to their receptor complexes and downstream signaling activity.

Control of signaling molecule range during developmental patterning

Tissue patterning, through the concerted activity of a small number of signaling pathways, is critical to embryonic development. While patterning can involve signaling between neighbouring cells, in other contexts signals act over greater distances by traversing complex cellular landscapes to instruct the fate of distant cells. In this review, we explore different strategies adopted by cells to modulate signaling molecule range to allow correct patterning. We describe mechanisms for restricting signaling range and highlight how such short-range signaling can be exploited to not only control the fate of adjacent cells, but also to generate graded signaling within a field of cells. Other strategies include modulation of signaling molecule action by tissue architectural properties and the use of cellular membranous structures, such as signaling filopodia and exosomes, to actively deliver signaling ligands to target cells. Signaling filopodia can also be deployed to reach out and collect particular signals, thereby precisely controlling their site of action.

Differences in the secretion and transport of Wnt proteins

During the last three decades, our understanding about Wnt signaling has progressed greatly, especially with regards to the molecular mechanism of intracellular transmission of this signaling, as well as its physiological roles. In parallel, the molecular nature of Wnt proteins has gradually but surely been clarified. Wnt proteins are post-translationaly modified with fatty acid and glycosaminoglycans, resulting in constraint of the 3D structure and behavior of the proteins. Specific binding proteins or extracellular vesicles, which appear to shield the lipid moiety from the aquatic environment, enable Wnt proteins to be transported in the extracellular space. Equally, Wnt-interacting proteins in the extracellular space, including heparan sulfate proteoglycan, are also involved in its spreading. Recent studies also show that intercellular transmission of Wnt proteins occurs by cell migration and extension of cell protrusions. Here, we will show the molecular and cellular bases of the trafficking of Wnt proteins and discuss questions that remain to be answered.

Signaling by Cellular Protrusions: Keeping the Conversation Private

Information exchange between different cells makes multicellular life possible. Signaling between cells can occur over long distances, as in the case of hormone signaling, or it can take place over short distances between immediately juxtaposed neighbors, as in the case of stem cell-niche signaling. The ability of signal-sending and -receiving cells to communicate with one another in a specific manner is of paramount importance in the proper development and function of tissues. Growing evidence indicates that different cellular protrusions help to achieve specificity in signaling that occurs between distinct cell types. Here, we focus on new roles for cellular protrusions in cell-to-cell communication, drawing special attention to how stem cells use specialized extensions to promote reception of self-renewing signals emanating from the niche.

Noggin4 is a long-range inhibitor of Wnt8 signalling that regulates head development in Xenopus laevis

Noggin4 is a Noggin family secreted protein whose molecular and physiological functions remain unknown. In this study, we demonstrate that in contrast to other Noggins, Xenopus laevis Noggin4 cannot antagonise BMP signalling; instead, it specifically binds to Wnt8 and inhibits the Wnt/β -catenin pathway. Live imaging demonstrated that Noggin4 diffusivity in embryonic tissues significantly exceeded that of other Noggins. Using the Fluorescence Recovery After Photobleaching (FRAP) assay and mathematical modelling, we directly estimated the affinity of Noggin4 for Wnt8 in living embryos and determined that Noggin4 fine-tune the Wnt8 posterior-to-anterior gradient. Our results suggest a role for Noggin4 as a unique, freely diffusing, long-range inhibitor of canonical Wnt signalling, thus explaining its ability to promote head development.

Signaling filopodia in vertebrate embryonic development

Next to classical diffusion-based models, filopodia-like cellular protrusions have been proposed to mediate long range signaling events and morphogen gradient formation during communication between distant cells. An increasing wealth of data indicates that in spite of variable characteristics of signaling filopodia in different biological contexts, they represent a paradigm of intercellular crosstalk which is presently being unraveled in a growing literature. Here, we summarize recent advances in investigating the morphology, cellular basis and function of signaling filopodia, with focus on their role during embryonic development in vertebrates.

Role of cytonemes in Wnt transport

Wnt signaling regulates a broad variety of processes during embryonic development and disease. A hallmark of the Wnt signaling pathway is the formation of concentration gradients by Wnt proteins across responsive tissues, which determines cell fate in invertebrates and vertebrates. To fulfill its paracrine function, trafficking of the Wnt morphogen from an origin cell to a recipient cell must be tightly regulated. A variety of models have been proposed to explain the extracellular transport of these lipid-modified signaling proteins in the aqueous extracellular space; however, there is still considerable debate with regard to which mechanisms allow the precise distribution of ligand in order to generate a morphogenetic gradient within growing tissue. Recent evidence suggests that Wnt proteins are distributed along signaling filopodia during vertebrate and invertebrate embryogenesis. Cytoneme-mediated transport has profound impact on our understanding of how Wnt signaling propagates through tissues and allows the formation of a precise ligand distribution in the recipient tissue during embryonic growth. In this Commentary, we review extracellular trafficking mechanisms for Wnt proteins and discuss the growing evidence of cytoneme-based Wnt distribution in development and stem cell biology. We will also discuss their implication for Wnt signaling in the formation of the Wnt morphogenetic gradient during tissue patterning.

Special Morphological Features at the Interface of the Renal Stem/Progenitor Cell Niche Force to Reinvestigate Transport of Morphogens During Nephron Induction

Formation of a nephron depends on reciprocal signaling of different morphogens between epithelial and mesenchymal cells within the renal stem/progenitor cell niche. Previously, it has been surmised that a close proximity exists between both involved cell types and that morphogens are transported between them by diffusion. However, actual morphological data illustrate that mesenchymal and epithelial stem/progenitor cell bodies are separated by a striking interface. Special fixation of specimens by glutaraldehyde (GA) solution including cupromeronic blue, ruthenium red, or tannic acid for electron microscopy depicts that the interface is not void but filled in extended areas by textured extracellular matrix. Surprisingly, projections of mesenchymal cells cross the interface to contact epithelial cells. At those sites the plasma membranes of a mesenchymal and an epithelial cell are connected via tunneling nanotubes. Regarding detected morphological features in combination with involved morphogens, their transport cannot longer be explained solely by diffusion. Instead, it has to be sorted according to biophysical properties of morphogens and to detected environment. Thus, the new working hypothesis is that morphogens with good solubility such as glial cell line-derived neurotrophic factor (GDNF) or fibroblast growth factors (FGFs) are transported by diffusion. Morphogens with minor solubility such as bone morphogenetic proteins (BMPs) are secreted and stored for delivery on demand in illustrated extracellular matrix. In contrast, morphogens with poor solubility such as Wnts are transported in mesenchymal cell projections along the plasma membrane or via illustrated tunneling nanotubes. However, the presence of an intercellular route between mesenchymal and epithelial stem/progenitor cells by tunneling nanotubes also makes it possible that all morphogens are transported this way.

Long-distance communication by specialized cellular projections during pigment pattern development and evolution

Changes in gene activity are essential for evolutionary diversification. Yet, elucidating the cellular behaviors that underlie modifications to adult form remains a profound challenge. We use neural crest-derived adult pigmentation of zebrafish and pearl danio to uncover cellular bases for alternative pattern states. We show that stripes in zebrafish require a novel class of thin, fast cellular projection to promote Delta-Notch signaling over long distances from cells of the xanthophore lineage to melanophores. Projections depended on microfilaments and microtubules, exhibited meandering trajectories, and stabilized on target cells to which they delivered membraneous vesicles. By contrast, the uniformly patterned pearl danio lacked such projections, concomitant with Colony stimulating factor 1-dependent changes in xanthophore differentiation that likely curtail signaling available to melanophores. Our study reveals a novel mechanism of cellular communication, roles for differentiation state heterogeneity in pigment cell interactions, and an unanticipated morphogenetic behavior contributing to a striking difference in adult form.

DOI:http://dx.doi.org/10.7554/eLife.12401.001

Animals have very different patterns of skin pigmentation, and these patterns can be important for survival and reproduction. Zebrafish, for example, have horizontal dark and light stripes along their bodies, while a closely related fish called the pearl danio has an almost uniform pattern.

The dark stripes of the zebrafish contain cells called melanophores, while the lighter regions contain two other types of cells known as xanthophores and iridophores. These pigment cell types interact with each other to create stripes. The iridophores establish the lighter stripes and specify the position and orientation of the dark stripes. They also produce a protein called Csf1, which allows the xanthophores to mature. As the stripes form, melanophores present in lighter stripes move into nearby dark stripes. Pearl danios also contain these three types of pigment cells, but these cells remain intermingled giving the fish their uniform color.

Eom et al. have now used microscopy to image pigment cells in zebrafish and pearl danio to uncover how interactions between these cells differ in species with different pigment patterns. The technique involved tagging pigment cells with fluorescent markers and using time-lapse imaging to track them during the formation of the adult pigmentation pattern.

The experiments show that stripes form in zebrafish because the cells that make the xanthophores form long, thin projections that extend to neighboring melanophores. These so-called ‘airinemes’ deliver materials to melanophores and help to clear the melanophores from interstripe regions, partly by activating a cell communication pathway called Delta-Notch signaling. These cell projections are mostly absent from the cells that make xanthophores in the pearl danio due to differences in when Csf1 is produced. This alters the timing of when the xanthophores develop, leading to the loss of long-distance airineme signaling.

Eom et al.’s findings identify a new way in which cells can communicate and an unanticipated cell behavior that contributes to a striking difference in the pigmentation patterns of zebrafish and pearl danio. Future studies should further our understanding of these unique projections and reveal whether they are produced by other types of cells.

DOI:http://dx.doi.org/10.7554/eLife.12401.002

Matters of context guide future research in TGFβ superfamily signaling

The highly conserved wiring of the SMAD-dependent transforming growth factor β (TGFβ) superfamily signaling pathway has been mapped over the last 20 years after molecular discovery of its component parts. Numerous alternative TGFβ-activated signaling pathways that elicit SMAD-independent biological responses also exist. However, the molecular mechanisms responsible for the renowned context dependency of TGFβ signaling output remains an active and often confounding area of research, providing a prototype relevant to regulation of other signaling pathways. Highlighting discoveries presented at the 9th FASEB meeting, The TGFβ Superfamily: Signaling in Development and Disease (July 12-17th 2015 in Snowmass, Colorado), this Review outlines research into the rich contextual nature of TGFβ signaling output and offers clues for therapeutic advances.

Filopodia-based Wnt transport during vertebrate tissue patterning

Paracrine Wnt/β-catenin signalling is important during developmental processes, tissue regeneration and stem cell regulation. Wnt proteins are morphogens, which form concentration gradients across responsive tissues. Little is known about the transport mechanism for these lipid-modified signalling proteins in vertebrates. Here we show that Wnt8a is transported on actin-based filopodia to contact responding cells and activate signalling during neural plate formation in zebrafish. Cdc42/N-Wasp regulates the formation of these Wnt-positive filopodia. Enhanced formation of filopodia increases the effective signalling range of Wnt by facilitating spreading. Consistently, reduction in filopodia leads to a restricted distribution of the ligand and a limited signalling range. Using a simulation, we provide evidence that such a short-range transport system for Wnt has a long-range signalling function. Indeed, we show that a filopodia-based transport system for Wnt8a controls anteroposterior patterning of the neural plate during vertebrate gastrulation.

Paracrine signaling mediated at cell-cell contacts

Recent findings in several organ systems show that cytoneme-mediated signaling transports signaling proteins along cellular extensions and targets cell-to-cell exchanges to synaptic contacts. This mechanism of paracrine signaling may be a general one that is used by many (or all) cell types in many (or all) organs. We briefly review these findings in this perspective. We also describe the properties of several signaling systems that have previously been interpreted to support a passive diffusion mechanism of signaling protein dispersion, but can now be understood in the context of the cytoneme mechanism. Also watch the Video Abstract.

Live imaging of Xwnt5A-ROR2 complexes

Secreted molecules of the Wnt family regulate key decisions in embryogenesis and adult tissue homeostasis by activating a complex network of Wnt signaling pathways. Although the different branches of Wnt signaling have been studied for more than 25 years, fluorophore tagged constructs for live cell imaging of Wnt molecules activating the Wnt/β-catenin pathway have become available only recently. We have generated a fluorophore tagged Wnt construct of the Xenopus Wnt5a protein (Xwnt5A) with the enhanced green fluorescent protein (EGFP), Xwnt5A-EGFP. This construct activates non-canonical Wnt pathways in an endocytosis dependent manner and is capable of compensating for the loss of endogenous Xwnt5A in Xenopus embryos. Strikingly, non-canonical Wnt pathway activation was restricted to short-range signaling while an inhibitory effect was observed in transwell cell cultures taken as long-range signaling model sytem. We used our Xwnt5A-EGFP construct to analyze in vivo binding of Wnt5A to its co-receptor ROR2 on the microscopic and on the molecular level. On the microscopic level, Xwnt5A-EGFP clusters in the membrane and recruits ROR2-mCherry to these clusters. Applying dual-colour dual-focus line-scanning fluorescence correlation spectroscopy on dorsal marginal zone explants, we identified membrane tethered Xwnt5A-EGFP molecules binding to ROR2-mCherry molecules. Our data favour a model, in which membrane-tethered Wnt-5A recruits ROR2 to form large ligand/receptor clusters and signals in an endocytosis-dependent manner.

Specialized filopodia: at the 'tip' of morphogen transport and vertebrate tissue patterning

For over a century, biologists have strived to unravel the mechanisms that establish how cells are informed of their position in the embryo and differentiate to give rise to complex organs and structures. However, the historical idea that one predominant mode of ligand transport, largely accounted for by free diffusion, can explain how all signaling molecules, known as morphogens, control tissue patterning has greatly hindered our ability to fully appreciate the complexities driving the delivery and reception of signaling molecules at a distance. In reality, a cell's shape, morphology, and location change continuously as development progresses. Thus, cellular context poses distinct challenges for morphogen transport in each unique cellular environment. Emerging studies reveal that some cells overcome such obstacles in an unexpected manner: via long, cellular projections, or specialized filopodia, that link distant cells and traffic signaling components. Here, we will review recent findings describing specialized filopodia and discuss the potential mechanisms and implications for filopodia-based long-range cell signaling and communication, particularly within the developing vertebrate embryo.