Tunnelling nanotube formation is driven by Eps8/IRSp53-dependent linear actin polymerization

Tunnelling nanotubes (TNTs) connect distant cells and mediate cargo transfer for intercellular communication in physiological and pathological contexts. How cells generate these actin-mediated protrusions to span lengths beyond those attainable by canonical filopodia remains unknown. Through a combination of micropatterning, microscopy, and optical tweezer-based approaches, we demonstrate that TNTs formed through the outward extension of actin achieve distances greater than the mean length of filopodia and that branched Arp2/3-dependent pathways attenuate the extent to which actin polymerizes in nanotubes, thus limiting their occurrence. Proteomic analysis using epidermal growth factor receptor kinase substrate 8 (Eps8) as a positive effector of TNTs showed that, upon Arp2/3 inhibition, proteins enhancing filament turnover and depolymerization were reduced and Eps8 instead exhibited heightened interactions with the inverted Bin/Amphiphysin/Rvs (I-BAR) domain protein IRSp53 that provides a direct connection with linear actin polymerases. Our data reveals how common protrusion players (Eps8 and IRSp53) form tunnelling nanotubes, and that when competing pathways overutilizing such proteins and monomeric actin in Arp2/3 networks are inhibited, processes promoting linear actin growth dominate to favour tunnelling nanotube formation.

Cytonemes coordinate asymmetric signaling and organization in the Drosophila muscle progenitor niche

Asymmetric signaling and organization in the stem-cell niche determine stem-cell fates. Here, we investigate the basis of asymmetric signaling and stem-cell organization using the Drosophila wing-disc that creates an adult muscle progenitor (AMP) niche. We show that AMPs extend polarized cytonemes to contact the disc epithelial junctions and adhere themselves to the disc/niche. Niche-adhering cytonemes localize FGF-receptor to selectively adhere to the FGF-producing disc and receive FGFs in a contact-dependent manner. Activation of FGF signaling in AMPs, in turn, reinforces disc-specific cytoneme polarity/adhesion, which maintains their disc-proximal positions. Loss of cytoneme-mediated adhesion promotes AMPs to lose niche occupancy and FGF signaling, occupy a disc-distal position, and acquire morphological hallmarks of differentiation. Niche-specific AMP organization and diversification patterns are determined by localized expression and presentation patterns of two different FGFs in the wing-disc and their polarized target-specific distribution through niche-adhering cytonemes. Thus, cytonemes are essential for asymmetric signaling and niche-specific AMP organization.

The Wnt Pathway: From Signaling Mechanisms to Synthetic Modulators

The Wnt pathway is central to a host of developmental and disease-related processes. The remarkable conservation of this intercellular signaling cascade throughout metazoan lineages indicates that it coevolved with multicellularity to regulate the generation and spatial arrangement of distinct cell types. By regulating cell fate specification, mitotic activity, and cell polarity, Wnt signaling orchestrates development and tissue homeostasis, and its dysregulation is implicated in developmental defects, cancer, and degenerative disorders. We review advances in our understanding of this key pathway, from Wnt protein production and secretion to relay of the signal in the cytoplasm of the receiving cell. We discuss the evolutionary history of this pathway as well as endogenous and synthetic modulators of its activity. Finally, we highlight remaining gaps in our knowledge of Wnt signal transduction and avenues for future research. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Regulation of Wnt distribution and function by Drosophila glypicans

The extracellular distribution of secreted Wnt proteins is crucial for their ability to induce a response in target cells at short and long ranges to ensure proper development. Wnt proteins are evolutionarily conserved ligands that are lipid-modified, and their hydrophobic nature interferes with their solubility in the hydrophilic extracellular environment. This raises the question of how Wnt proteins spread extracellularly despite their lipid modifications, which are essential for both their secretion and function. Seminal studies on Drosophila Wingless (Wg), a prototypical Wnt, have discovered multiple mechanisms by which Wnt proteins spread. A central theme emerges from these studies: the Wnt lipid moiety is shielded from the aqueous environment, allowing the ligands to spread and remain viable for signaling. Wnt distribution in vivo is primarily facilitated by glypicans, which are cell-surface heparan sulfate proteoglycans, and recent studies have further provided mechanistic insight into how glypicans facilitate Wnt distribution. In this Review, we discuss the many diverse mechanisms of Wnt distribution, with a particular focus on glypican-mediated mechanisms.

Glypican 4 mediates Wnt transport between germ layers via signaling filopodia

Glypicans influence signaling pathways by regulating morphogen trafficking and reception. However, the underlying mechanisms in vertebrates are poorly understood. In zebrafish, Glypican 4 (Gpc4) is required for convergence and extension (C&E) of both the mesoderm and endoderm. Here, we show that transgenic expression of GFP-Gpc4 in the endoderm of gpc4 mutants rescued C&E defects in all germ layers. The rescue of mesoderm was likely mediated by Wnt5b and Wnt11f2 and depended on signaling filopodia rather than on cleavage of the Gpc4 GPI anchor. Gpc4 bound both Wnt5b and Wnt11f2 and regulated formation of the filopodia that transport Wnt5b and Wnt11f2 to neighboring cells. Moreover, this rescue was suppressed by blocking signaling filopodia that extend from endodermal cells. Thus, GFP-Gpc4–labeled protrusions that emanated from endodermal cells transported Wnt5b and Wnt11f2 to other germ layers, rescuing the C&E defects caused by a gpc4 deficiency. Our study reveals a new mechanism that could explain in vivo morphogen distribution involving Gpc4.

Extracellular WNTs: Trafficking, Exosomes, and Ligand-Receptor Interaction

WNT signaling is a key developmental pathway in tissue organization. A recent focus of research is the secretion of WNT proteins from source cells. Research over the past decade on how WNTs are produced and released into the extracellular space has unravelled very specific control mechanisms in the early secretory pathway, specialized trafficking routes, and redundant forms of packaging for delivery to target cells. In this review I discuss the findings that WNT proteins have been found on extracellular vesicles (EVs) such as exosomes and possible functional implications. There is an ongoing debate in the WNT signaling field whether EV are relevant in vivo and can fulfill specific functions, also fueled by the general preconception of EV secretion as cellular garbage disposal. As part of the EV research community, I want to give an overview of what we know and don't know about WNT secretion on EVs and offer a more unifying model that can explain current discrepancies in observations regarding WNT secretion.

Cryo-EM structure of human Wntless in complex with Wnt3a

Wntless (WLS), an evolutionarily conserved multi-pass transmembrane protein, is essential for secretion of Wnt proteins. Wnt-triggered signaling pathways control many crucial life events, whereas aberrant Wnt signaling is tightly associated with many human diseases including cancers. Here, we report the cryo-EM structure of human WLS in complex with Wnt3a, the most widely studied Wnt, at 2.2 Å resolution. The transmembrane domain of WLS bears a GPCR fold, with a conserved core cavity and a lateral opening. Wnt3a interacts with WLS at multiple interfaces, with the lipid moiety on Wnt3a traversing a hydrophobic tunnel of WLS transmembrane domain and inserting into membrane. A β-hairpin of Wnt3a containing the conserved palmitoleoylation site interacts with WLS extensively, which is crucial for WLS-mediated Wnt secretion. The flexibility of the Wnt3a loop/hairpin regions involved in the multiple binding sites indicates induced fit might happen when Wnts are bound to different binding partners. Our findings provide important insights into the molecular mechanism of Wnt palmitoleoylation, secretion and signaling.

Regulated delivery controls Drosophila Hedgehog, Wingless, and Decapentaplegic signaling

Morphogen signaling proteins disperse across tissues to activate signal transduction in target cells. We investigated dispersion of Hedgehog (Hh), Wnt homolog Wingless (Wg), and Bone morphogenic protein homolog Decapentaplegic (Dpp) in the Drosophila wing imaginal disc. We discovered that delivery of Hh, Wg, and Dpp to their respective targets is regulated. We found that <5% of Hh and <25% of Wg are taken up by disc cells and activate signaling. The amount of morphogen that is taken up and initiates signaling did not change when the level of morphogen expression was varied between 50 and 200% (Hh) or 50 and 350% (Wg). Similar properties were observed for Dpp. We analyzed an area of 150 μm×150 μm that includes Hh-responding cells of the disc as well as overlying tracheal cells and myoblasts that are also activated by disc-produced Hh. We found that the extent of signaling in the disc was unaffected by the presence or absence of the tracheal and myoblast cells, suggesting that the mechanism that disperses Hh specifies its destinations to particular cells, and that target cells do not take up Hh from a common pool.

Axon guidance at the spinal cord midline-A live imaging perspective

During neural circuit formation, axons navigate several choice points to reach their final target. At each one of these intermediate targets, growth cones need to switch responsiveness from attraction to repulsion in order to move on. Molecular mechanisms that allow for the precise timing of surface expression of a new set of receptors that support the switch in responsiveness are difficult to study in vivo. Mostly, mechanisms are inferred from the observation of snapshots of many different growth cones analyzed in different preparations of tissue harvested at distinct time points. However, to really understand the behavior of growth cones at choice points, a single growth cone should be followed arriving at and leaving the intermediate target. Existing ex vivo preparations, like cultures of an "open-book" preparation of the spinal cord have been successfully used to study floor plate entry and exit, but artifacts prevent the analysis of growth cone behavior at the floor plate exit site. Here, we describe a novel spinal cord preparation that allows for live imaging of individual axons during navigation in their intact environment. When comparing growth cone behavior in our ex vivo system with snapshots from in vivo navigation, we do not see any differences. The possibility to observe the dynamics of single growth cones navigating their intermediate target allows for measuring growth speed, changes in morphology, or aberrant behavior, like stalling and wrong turning. Moreover, observation of the intermediate target-the floor plate-revealed its active participation and interaction with commissural axons during midline crossing.

Sensory Neuroblast Quiescence Depends on Vascular Cytoneme Contacts and Sensory Neuronal Differentiation Requires Initiation of Blood Flow

In many organs, stem cell function depends on communication with their niche partners. Cranial sensory neurons develop in close proximity to blood vessels; however, whether vasculature is an integral component of their niches is yet unknown. Here, two separate roles for vasculature in cranial sensory neurogenesis in zebrafish are uncovered. The first involves precise spatiotemporal endothelial-neuroblast cytoneme contacts and Dll4-Notch signaling to restrain neuroblast proliferation. The second, instead, requires blood flow to trigger a transcriptional response that modifies neuroblast metabolic status and induces sensory neuron differentiation. In contrast, no role of sensory neurogenesis in vascular development is found, suggesting unidirectional signaling from vasculature to sensory neuroblasts. Altogether, we demonstrate that the cranial vasculature constitutes a niche component of the sensory ganglia that regulates the pace of their growth and differentiation dynamics.

Peering into tunneling nanotubes-The path forward

The identification of Tunneling Nanotubes (TNTs) and TNT-like structures signified a critical turning point in the field of cell-cell communication. With hypothesized roles in development and disease progression, TNTs' ability to transport biological cargo between distant cells has elevated these structures to a unique and privileged position among other mechanisms of intercellular communication. However, the field faces numerous challenges-some of the most pressing issues being the demonstration of TNTs in vivo and understanding how they form and function. Another stumbling block is represented by the vast disparity in structures classified as TNTs. In order to address this ambiguity, we propose a clear nomenclature and provide a comprehensive overview of the existing knowledge concerning TNTs. We also discuss their structure, formation-related pathways, biological function, as well as their proposed role in disease. Furthermore, we pinpoint gaps and dichotomies found across the field and highlight unexplored research avenues. Lastly, we review the methods employed to date and suggest the application of new technologies to better understand these elusive biological structures.

Vangl2 promotes the formation of long cytonemes to enable distant Wnt/β-catenin signaling

Wnt signaling regulates cell proliferation and cell differentiation as well as migration and polarity during development. However, it is still unclear how the Wnt ligand distribution is precisely controlled to fulfil these functions. Here, we show that the planar cell polarity protein Vangl2 regulates the distribution of Wnt by cytonemes. In zebrafish epiblast cells, mouse intestinal telocytes and human gastric cancer cells, Vangl2 activation generates extremely long cytonemes, which branch and deliver Wnt protein to multiple cells. The Vangl2-activated cytonemes increase Wnt/β-catenin signaling in the surrounding cells. Concordantly, Vangl2 inhibition causes fewer and shorter cytonemes to be formed and reduces paracrine Wnt/β-catenin signaling. A mathematical model simulating these Vangl2 functions on cytonemes in zebrafish gastrulation predicts a shift of the signaling gradient, altered tissue patterning, and a loss of tissue domain sharpness. We confirmed these predictions during anteroposterior patterning in the zebrafish neural plate. In summary, we demonstrate that Vangl2 is fundamental to paracrine Wnt/β-catenin signaling by controlling cytoneme behaviour.

Generation of extracellular morphogen gradients: the case for diffusion

Cells within developing tissues rely on morphogens to assess positional information. Passive diffusion is the most parsimonious transport model for long-range morphogen gradient formation but does not, on its own, readily explain scaling, robustness and planar transport. Here, we argue that diffusion is sufficient to ensure robust morphogen gradient formation in a variety of tissues if the interactions between morphogens and their extracellular binders are considered. A current challenge is to assess how the affinity for extracellular binders, as well as other biophysical and cell biological parameters, determines gradient dynamics and shape in a diffusion-based transport system. Technological advances in genome editing, tissue engineering, live imaging and in vivo biophysics are now facilitating measurement of these parameters, paving the way for mathematical modelling and a quantitative understanding of morphogen gradient formation and modulation.

First-passage processes and the target-based accumulation of resources

A random search for one or more targets in a bounded domain occurs widely in nature, with examples ranging from animal foraging to the transport of vesicles within cells. Most theoretical studies take a searcher-centric viewpoint, focusing on the first passage time (FTP) problem to find a target. This single search-and-capture event then triggers a downstream process or provides the searcher with some resource such as food. In this paper we take a target-centric viewpoint by considering the accumulation of resources in one or more targets due to multiple rounds of search-and-capture events combined with resource degradation; whenever a searcher finds a target it delivers a resource packet to the target, after which it escapes and returns to its initial position. The searcher is then resupplied with cargo and a new search process is initiated after a random delay. It has previously been shown how queuing theory can be used to derive general expressions for the steady-state mean and variance of the resulting resource distributions. Here we apply the theory to some classical FPT problems involving diffusion in simple geometries with absorbing boundaries, including concentric spheres, wedge domains, and branching networks. In each case, we determine how the resulting Fano factor depends on the degradation rate, the delay distribution, and various geometric parameters. We thus establish that the Fano factor can deviate significantly from Poisson statistics and exhibits a nontrivial dependence on model parameters, including nonmonotonicity and crossover behavior. This indicates the nontrivial nature of the higher-order statistics of resource accumulation.

Structural Basis of WLS/Evi-Mediated Wnt Transport and Secretion

Wnts are evolutionarily conserved ligands that signal at short range to regulate morphogenesis, cell fate, and stem cell renewal. The first and essential steps in Wnt secretion are their O-palmitoleation and subsequent loading onto the dedicated transporter Wntless/evenness interrupted (WLS/Evi). We report the 3.2 Å resolution cryogenic electron microscopy (cryo-EM) structure of palmitoleated human WNT8A in complex with WLS. This is accompanied by biochemical experiments to probe the physiological implications of the observed association. The WLS membrane domain has close structural homology to G protein-coupled receptors (GPCRs). A Wnt hairpin inserts into a conserved hydrophobic cavity in the GPCR-like domain, and the palmitoleate protrudes between two helices into the bilayer. A conformational switch of highly conserved residues on a separate Wnt hairpin might contribute to its transfer to receiving cells. This work provides molecular-level insights into a central mechanism in animal body plan development and stem cell biology.

Wnt signalling and endocytosis: Mechanisms, controversies and implications for stress responses

Wnt signalling is one of a few conserved pathways that control diverse aspects of development and morphogenesis in all metazoan species. Endocytosis is a key mechanism that regulates the secretion and graded extracellular distribution of Wnt glycoproteins from the source cells, as well as Wnt signal transduction in the receiving cells. However, controversies exist regarding the requirement of clathrin-dependent endocytosis in Wnt signalling. Various lines of evidence from recent studies suggest that Wnt-β-catenin signalling is also involved in the regulation of cellular stress responses in adulthood, a role that is beyond its canonical functions in animal development. In this review, we summarise recent advances in the molecular and cellular mechanisms by which endocytosis modulates Wnt signalling. We also discuss how Wnt signalling could be repurposed to regulate mitochondrial stress response in the nematode Caenorhabditis elegans.

Diffusion vs. direct transport in the precision of morphogen readout

Morphogen profiles allow cells to determine their position within a developing organism, but not all morphogen profiles form by the same mechanism. Here, we derive fundamental limits to the precision of morphogen concentration sensing for two canonical mechanisms: the diffusion of morphogen through extracellular space and the direct transport of morphogen from source cell to target cell, for example, via cytonemes. We find that direct transport establishes a morphogen profile without adding noise in the process. Despite this advantage, we find that for sufficiently large values of profile length, the diffusion mechanism is many times more precise due to a higher refresh rate of morphogen molecules. We predict a profile lengthscale below which direct transport is more precise, and above which diffusion is more precise. This prediction is supported by data from a wide variety of morphogens in developing Drosophila and zebrafish.

Human CD8+ T Cells Release Extracellular Traps Co-Localized With Cytotoxic Vesicles That Are Associated With Lesion Progression and Severity in Human Leishmaniasis

Cell death plays a fundamental role in mounting protective and pathogenic immunity. Etosis is a cell death mechanism defined by the release of extracellular traps (ETs), which can foster inflammation and exert microbicidal activity. While etosis is often associated with innate cells, recent studies showed that B cells and CD4+ T cells can release ETs. Here we investigate whether CD8+ T cells can also release ETs, which might be related to cytotoxicity and tissue pathology. To these ends, we first employed an in vitro system stimulating human CD8+ T cells isolated from healthy volunteers with anti-CD3/anti-CD28. Using time-frame video, confocal and electron microscopy, we demonstrate that human CD8+ T cells release ETs upon stimulation (herein LETs – lymphocyte extracellular traps), which display unique morphology and functional characteristics. CD8+ T cell-derived LETs form long strands that co-localize with CD107a, a marker of vesicles containing cytotoxic granules. In addition, these structures connect the LET-releasing cell to other neighboring cells, often resulting in cell death. After demonstrating the release of LETs by human CD8+ T cells in vitro, we went on to study the occurrence of CD8-derived LETs in a human disease setting. Thus, we evaluated the occurrence of CD8-derived LETs in lesions from patients with human tegumentary leishmaniasis, where CD8+ T cells play a key role in mediating pathology. In addition, we evaluated the association of these structures with the intensity of the inflammatory infiltrate in early and late cutaneous, as well as in mucosal leishmaniasis lesions. We demonstrated that progression and severity of debilitating and mutilating forms of human tegumentary leishmaniasis are associated with the frequency of CD8+ T cells in etosis, as well as the occurrence of CD8-derived LETs carrying CD107a+ vesicles in the lesions. We propose that CD8+ T cell derived LETs may serve as a tool for delivering cytotoxic vesicles to distant target cells, providing insights into mechanisms of CD8+ T cell mediated pathology.

Target competition for resources under multiple search-and-capture events with stochastic resetting

We develop a general framework for analysing the distribution of resources in a population of targets under multiple independent search-and-capture events. Each event involves a single particle executing a stochastic search that resets to a fixed location x r at a random sequence of times. Whenever the particle is captured by a target, it delivers a packet of resources and then returns to x r , where it is reloaded with cargo and a new round of search and capture begins. Using renewal theory, we determine the mean number of resources in each target as a function of the splitting probabilities and unconditional mean first passage times of the corresponding search process without resetting. We then use asymptotic PDE methods to determine the effects of resetting on the distribution of resources generated by diffusive search in a bounded two-dimensional domain with N small interior targets. We show that slow resetting increases the total number of resources M tot across all targets provided that ∑ j = 1 N G ( x r , x j ) < 0 , where G is the Neumann Green's function and x j is the location of the j-th target. This implies that M tot can be optimized by varying r. We also show that the k-th target has a competitive advantage if ∑ j = 1 N G ( x r , x j ) > N G ( x r , x k ) .

Search processes with stochastic resetting and multiple targets

Search processes with stochastic resetting provide a general theoretical framework for understanding a wide range of naturally occurring phenomena. Most current models focus on the first-passage-time problem of finding a single target in a given search domain. Here we use a renewal method to derive general expressions for the splitting probabilities and conditional mean first passage times (MFPTs) in the case of multiple targets. Our analysis also incorporates the effects of delays arising from finite return times and refractory periods. Carrying out a small-r expansion, where r is the mean resetting rate, we obtain general conditions for when resetting increases the splitting probability or reduces the conditional MFPT to a particular target. This also depends on whether π_{tot}=1 or π_{tot}<1, where π_{tot} is the probability that the particle is eventually absorbed by one of the targets in the absence of resetting. We illustrate the theory by considering two distinct examples. The first consists of an actin-rich cell filament (cytoneme) searching along a one-dimensional array of target cells, a problem for which the splitting probabilities and MFPTs can be calculated explicitly. In particular, we highlight how the resetting rate plays an important role in shaping the distribution of splitting probabilities along the array. The second example involves a search process in a three-dimensional bounded domain containing a set of N small interior targets. We use matched asymptotics and Green's functions to determine the behavior of the splitting probabilities and MFPTs in the small-r regime. In particular, we show that the splitting probabilities and MFPTs depend on the "shape capacitance" of the targets.

Frizzled-Dependent Planar Cell Polarity without Secreted Wnt Ligands

Planar cell polarity (PCP) organizes the orientation of cellular protrusions and migratory activity within the tissue plane. PCP establishment involves the subcellular polarization of core PCP components. It has been suggested that Wnt gradients could provide a global cue that coordinates local PCP with tissue axes. Here, we dissect the role of Wnt ligands in the orientation of hairs of Drosophila wings, an established system for the study of PCP. We found that PCP was normal in quintuple mutant wings that rely solely on the membrane-tethered Wingless for Wnt signaling, suggesting that a Wnt gradient is not required. We then used a nanobody-based approach to trap Wntless in the endoplasmic reticulum, and hence prevent all Wnt secretion, specifically during the period of PCP establishment. PCP was still established. We conclude that, even though Wnt ligands could contribute to PCP, they are not essential, and another global cue must exist for tissue-wide polarization.

Queueing theory of search processes with stochastic resetting

We use queueing theory to develop a general framework for analyzing search processes with stochastic resetting, under the additional assumption that following absorption by a target, the particle (searcher) delivers a packet of resources to the target and the search process restarts at the reset point x_{r}. This leads to a sequence of search-and-capture events, whereby resources accumulate in the target under the combined effects of resource supply and degradation. Combining the theory of G/M/∞ queues with a renewal method for analyzing resetting processes, we derive general expressions for the mean and variance of the number of resource packets within the target at steady state. These expressions apply to both exponential and nonexponential resetting protocols and take into account delays arising from various factors such as finite return times, refractory periods, and delays due to the loading or unloading of resources. In the case of exponential resetting, we show how the resource statistics can be expressed in terms of the MFPTs T_{r}(x_{r}) and T_{r+γ}(x_{r}), where r is the resetting rate and γ is the degradation rate. This allows us to derive various general results concerning the dependence of the mean and variance on the parameters r,γ. Our results are illustrated using several specific examples. Finally, we show how fluctuations can be reduced either by allowing the delivery of multiple packets that degrade independently or by having multiple independent searchers.

Heparan Sulfate Proteoglycan Clustering in Wnt Signaling and Dispersal

Wnt, a family of secreted signal proteins, serves diverse functions in animal development, stem cell systems, and carcinogenesis. Although Wnt is generally considered a morphogen, the mechanism by which Wnt ligands disperse is still debated. Heparan sulfate proteoglycans (HSPGs) are extracellular regulators involved in Wnt ligand dispersal. Drosophila genetics have revealed that HSPGs participate in accumulation and transport of Wnt ligands. Based on these findings, a "restricted diffusion" model, in which Wnt ligands are gradually transferred by repetitive binding and dissociation to HSPGs, has been proposed. Nonetheless, we recently found that HSPGs are not uniformly distributed, but are locally clustered on cell surfaces in Xenopus embryos. HSPGs with N-sulfo-rich HS chains and those with N-acetyl-rich unmodified HS chains form different clusters. Furthermore, endogenous Wnt8 ligands are discretely accumulated in a punctate fashion, colocalized with the N-sulfo-rich clusters. Based on these lines of evidence, here we reconsider the classical view of morphogen spreading controlled by HSPGs.

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.

Targeting Wnt Signaling for the Treatment of Gastric Cancer

The Wnt signaling pathway is evolutionarily conserved, regulating both embryonic development and maintaining adult tissue homeostasis. Wnt signaling controls several fundamental cell functions, including proliferation, differentiation, migration, and stemness. It therefore plays an important role in the epithelial homeostasis and regeneration of the gastrointestinal tract. Often, both hypo- or hyper-activation of the pathway due to genetic, epigenetic, or receptor/ligand alterations are seen in many solid cancers, such as breast, colorectal, gastric, and prostate. Gastric cancer (GC) is the fourth commonest cause of cancer worldwide and is the second leading cause of cancer-related death annually. Although the number of new diagnoses has declined over recent decades, prognosis remains poor, with only 15% surviving to five years. Geographical differences in clinicopathological features are also apparent, with epidemiological and genetic studies revealing GC to be a highly heterogeneous disease with phenotypic diversity as a result of etiological factors. The molecular heterogeneity associated with GC dictates that a single 'one size fits all' approach to management is unlikely to be successful. Wnt pathway dysregulation has been observed in approximately 50% of GC tumors and may offer a novel therapeutic target for patients who would otherwise have a poor outcome. This mini review will highlight some recent discoveries involving Wnt signaling in GC.

Fine intercellular connections in development: TNTs, cytonemes, or intercellular bridges?

Intercellular communication is a fundamental property of multicellular organisms, necessary for their adequate responses to changing environment. Tunneling nanotubes (TNTs) represent a novel means of intercellular communication being a long cell-to-cell conduit. TNTs are actively formed under a broad range of stresses and are also proposed to exist under physiological conditions. Development is a physiological condition of particular interest, as it requires fine coordination. Here we discuss whether protrusions shown to exist during embryonic development of different species could be TNTs or if they represent other types of cell structure, like cytonemes or intercellular bridges, that are suggested to play an important role in development.

Human myotubularin-related protein 9 regulates ER-to-Golgi trafficking and modulates WNT3A secretion

Regulation of phosphatidylinositol phosphates plays a crucial role in signal transduction, membrane trafficking or autophagy. Members of the myotubularin family of lipid phosphatases contribute to phosphoinositide metabolism by counteracting the activity of phosphoinositide kinases. The mechanisms determining their subcellular localization and targeting to specific membrane compartments are still poorly understood. We show here that the inactive phosphatase MTMR9 localizes to the intermediate compartment and to the Golgi apparatus and is able to recruit its active phosphatase partners MTMR6 and MTMR8 to these locations. Furthermore, MTMR8 and MTMR9 co-localize with the small GTPase RAB1A and regulate its localization. Loss of MTMR9 expression compromises the integrity of the Golgi apparatus and results in altered distribution of RAB1A and actin nucleation-promoting factor WHAMM. Loss or overexpression of MTMR9 leads to decreased rate of protein secretion. We demonstrate that secretion of physiologically relevant cargo exemplified by the WNT3A protein is affected after perturbation of MTMR9 levels.

The Wnt/Ca2+ pathway is involved in interneuronal communication mediated by tunneling nanotubes

Tunneling nanotubes (TNTs) are actin-based transient tubular connections that allow direct communication between distant cells. TNTs play an important role in several physiological (development, immunity, and tissue regeneration) and pathological (cancer, neurodegeneration, and pathogens transmission) processes. Here, we report that the Wnt/Ca2+ pathway, an intracellular cascade that is involved in actin cytoskeleton remodeling, has a role in TNT formation and TNT-mediated transfer of cargoes. Specifically, we found that Ca2+ /calmodulin-dependent protein kinase II (CaMKII), a transducer of the Wnt/Ca2+ pathway, regulates TNTs in a neuronal cell line and in primary neurons. We identified the β isoform of CaMKII as a key molecule in modulating TNT formation and transfer, showing that this depends on the actin-binding activity of the protein. Finally, we found that the transfer of vesicles and aggregated α-synuclein between primary neurons can be regulated by the activation of the Wnt/Ca2+ pathway. Our findings suggest that Wnt/Ca2+ pathway could be a novel promising target for therapies designed to impair TNT-mediated propagation of pathogens.

Cytonemes, Their Formation, Regulation, and Roles in Signaling and Communication in Tumorigenesis

Increasing evidence during the past two decades shows that cells interconnect and communicate through cytonemes. These cytoskeleton-driven extensions of specialized membrane territories are involved in cell–cell signaling in development, patterning, and differentiation, but also in the maintenance of adult tissue homeostasis, tissue regeneration, and cancer. Brain tumor cells in glioblastoma extend ultralong membrane protrusions (named tumor microtubes, TMs), which contribute to invasion, proliferation, radioresistance, and tumor progression. Here we review the mechanisms underlying cytoneme formation, regulation, and their roles in cell signaling and communication in epithelial cells and other cell types. Furthermore, we discuss the recent discovery of glial cytonemes in the Drosophila glial cells that alter Wingless (Wg)/Frizzled (Fz) signaling between glia and neurons. Research on cytoneme formation, maintenance, and cell signaling mechanisms will help to better understand not only physiological developmental processes and tissue homeostasis but also cancer progression.

The N-space Episenome unifies cellular information space-time within cognition-based evolution

Self-referential cellular homeostasis is maintained by the measured assessment of both internal status and external conditions based within an integrated cellular information field. This cellular field attachment to biologic information space-time coordinates environmental inputs by connecting the cellular senome, as the sum of the sensory experiences of the cell, with its genome and epigenome. In multicellular organisms, individual cellular information fields aggregate into a collective information architectural matrix, termed a N-space Episenome, that enables mutualized organism-wide information management. It is hypothesized that biological organization represents a dual heritable system constituted by both its biological materiality and a conjoining N-space Episenome. It is further proposed that morphogenesis derives from reciprocations between these inter-related facets to yield coordinated multicellular growth and development. The N-space Episenome is conceived as a whole cell informational projection that is heritable, transferable via cell division and essential for the synchronous integration of the diverse self-referential cells that constitute holobionts.

Robust Wnt signaling is maintained by a Wg protein gradient and Fz2 receptor activity in the developing Drosophila wing

Wnts are secreted proteins that regulate cell fate during development of all metazoans. Wnt proteins were proposed to spread over several cells to activate signaling directly at a distance. In the Drosophila wing epithelium, an extracellular gradient of the Wnt1 homolog Wingless (Wg) was observed extending over several cells away from producing cells. Surprisingly, however, it was also shown that a membrane-tethered Neurotactin-Wg fusion protein (NRT-Wg) can largely replace endogenous Wg, leading to proper patterning of the wing. Therefore, the functional range of Wg and whether Wg spreading is required for correct tissue patterning remains controversial. Here, by capturing secreted Wg on cells away from the source, we show that Wg acts over a distance of up to 11 cell diameters to induce signaling. Furthermore, cells located outside the reach of extracellular Wg depend on the Frizzled2 receptor to maintain signaling. Frizzled2 expression is increased in the absence of Wg secretion and is required to maintain signaling and cell survival in NRT-wg wing discs. Together, these results provide insight into the mechanisms by which robust Wnt signaling is achieved in proliferating tissues.

Cytonemes in development

Cell-cell communication is essential during the development of multicellular organisms. Specialized cell protrusions called cytonemes have been identified to exchange signals between cells that are vital for tissue development. Cytonemes can carry signalling components between distant cells and thus regulate the activity levels of the corresponding signalling pathways across entire tissues. This review summarizes the key findings on the formation and function of cytonemes in tissue development.

Impulsive signaling model of cytoneme-based morphogen gradient formation

Morphogen protein gradients play a vital role in regulating spatial pattern formation during development. The most commonly accepted mechanism of protein gradient formation involves the diffusion and degradation of morphogens from a localized source. However, there is growing experimental evidence for a direct cell-to-cell signaling mechanism via thin actin-rich cellular extensions known as cytonemes. Recent modeling studies of cytoneme-based morphogenesis in invertebrates ignore the discrete nature of vesicular transport along cytonemes, focusing on deterministic continuum models. In this paper, we develop an impulsive signaling model of morphogen gradient formation in invertebrates, which takes into account the discrete and stochastic nature of vesicular transport along cytonemes. We begin by solving a first passage time problem with sticky boundaries to determine the expected time to deliver a vesicle to a target cell, assuming that there is a 'nucleation' time for injecting the vesicle into the cytoneme. We then use queuing theory to analyze the impulsive model of morphogen gradient formation in the case of multiple cytonemes and multiple targets. In particular, we determine the steady-state mean and variance of the morphogen distribution across a one-dimensional array of target cells. The mean distribution recovers the spatially decaying morphogen gradient of previous deterministic models. However, the burst-like nature of morphogen transport can lead to Fano factors greater than unity across the array of cells, resulting in significant fluctuations at more distant target sites.

Imaging Cytonemes in Drosophila Embryos

Conserved morphogenetic signaling proteins disperse across tissues to generate signal and signaling gradients, which in turn are considered to assign positional coordinates to the recipient cells. Recent imaging studies in Drosophila model have provided evidence for a "direct-delivery" mechanism of signal dispersion that is mediated by specialized actin-rich signaling filopodia, named cytonemes. Cytonemes establish contact between the signal-producing and target cells to directly exchange and transport the morphogenetic proteins. Although an increasing amount of evidence supports the critical role of these specialized signaling structures, imaging these highly dynamic 200 nm-thin structures in the complex three-dimensional contour of living tissues is challenging. Here, we describe the imaging methods that we optimized for studying cytonemes in Drosophila embryos.

Essential long-range action of Wingless/Wnt in adult intestinal compartmentalization

Signal transduction activated by Wingless/Wnt ligands directs cell proliferation and fate specification in metazoans, and its overactivation underlies the development of the vast majority of colorectal cancers. In the conventional model, the secretion and movement of Wingless to cells distant from its source of synthesis are essential for long-range signaling in tissue patterning. However, this model was upended recently by an unanticipated finding: replacement of wild-type Drosophila Wingless with a membrane-tethered form produced viable adults with largely normal external morphology, which suggested that Wingless secretion and movement are dispensable for tissue patterning. Herein, we tested this foundational principle in the adult intestine, where Wingless signaling gradients coincide with all major boundaries between compartments. We find that the critical roles of Wingless during adult intestinal development, which include regulation of target gene activation, boundary formation, stem cell proliferation, epithelial cell fate specification, muscle differentiation, gut folding, and signaling crosstalk with the Decapentaplegic pathway, are all disrupted by Wingless tethering. These findings provide new evidence that supports the requirement for the direct, long-range action of Wingless in tissue patterning, with relevance for animal development, tissue homeostasis and Wnt-driven disease.

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.

Search-and-capture model of cytoneme-mediated morphogen gradient formation

Morphogen protein gradients play an essential role in the spatial regulation of patterning during embryonic development. The most commonly accepted mechanism of protein gradient formation involves the diffusion and degradation of morphogens from a localized source. Recently, an alternative mechanism has been proposed, which is based on cell-to-cell transport via thin actin-rich cellular extensions known as cytonemes. Very little is currently known about the precise nature of the contacts between cytonemes and their target cells. Important unresolved issues include how cytoneme tips find their targets, how they are stabilized at their contact sites, and how vesicles are transferred to a receiving cell and subsequently internalized. It has been hypothesized that cytonemes find their targets via a random search process based on alternating periods of retraction and growth, perhaps mediated by some chemoattractant. This is an actin-based analog of the search-and-capture model of microtubules of the mitotic spindle searching for cytochrome binding sites (kinetochores) prior to separation of cytochrome pairs. In this paper we develop a search-and-capture model of cytoneme-based morphogenesis, in which nucleating cytonemes from a source cell dynamically grow and shrink along the surface of a one-dimensional array of target cells until making contact with one of the target cells. We analyze the first-passage-time problem for making contact and then use this to explore the formation of morphogen gradients under the mechanism proposed for Wnt in vertebrates. That is, we assume that morphogen is localized at the tip of a growing cytoneme, which is delivered as a "morphogen burst" to a target cell when the cytoneme makes temporary contact with a target cell before subsequently retracting. We show how multiple rounds of search-and-capture, morphogen delivery, cytoneme retraction, and nucleation events lead to the formation of a morphogen gradient. We proceed by formulating the morphogen bursting model as a queuing process, analogous to the study of translational bursting in gene networks. In order to analyze the expected times for cytoneme contact, we introduce an efficient method for solving first-passage-time problems in the presence of sticky boundaries, which exploits some classical concepts from probability theory, namely, stopping times and the strong Markov property. We end the paper by demonstrating how this method simplifies previous analyses of a well-studied problem in cell biology, namely, the search-and-capture model of microtubule-kinetochore attachment. Although the latter is completely unrelated to cytoneme-based morphogenesis from a biological perspective, it shares many of the same mathematical elements.

Wnt traffic from endoplasmic reticulum to filopodia

Wnts are a family of secreted palmitoleated glycoproteins that play key roles in cell to cell communication during development and regulate stem cell compartments in adults. Wnt receptors, downstream signaling cascades and target pathways have been extensively studied while less is known about how Wnts are secreted and move from producing cells to receiving cells. We used the synchronization system called Retention Using Selective Hook (RUSH) to study Wnt trafficking from endoplasmic reticulum to Golgi and then to plasma membrane and filopodia in real time. Inhibition of porcupine (PORCN) or knockout of Wntless (WLS) blocked Wnt exit from the ER. Wnt-containing vesicles paused at sub-cortical regions of the plasma membrane before exiting the cell. Wnt-containing vesicles were associated with filopodia extending to adjacent cells. These data visualize and confirm the role of WLS and PORCN in ER exit of Wnts and support the role of filopodia in Wnt signaling.

Phenotypic analysis of Myo10 knockout (Myo10tm2/tm2) mice lacking full-length (motorized) but not brain-specific headless myosin X

We investigated the physiological functions of Myo10 (myosin X) using Myo10 reporter knockout (Myo10tm2) mice. Full-length (motorized) Myo10 protein was deleted, but the brain-specific headless (Hdl) isoform (Hdl-Myo10) was still expressed in homozygous mutants. In vitro, we confirmed that Hdl-Myo10 does not induce filopodia, but it strongly localized to the plasma membrane independent of the MyTH4-FERM domain. Filopodia-inducing Myo10 is implicated in axon guidance and mice lacking the Myo10 cargo protein DCC (deleted in colorectal cancer) have severe commissural defects, whereas MRI (magnetic resonance imaging) of isolated brains revealed intact commissures in Myo10tm2/tm2 mice. However, reminiscent of Waardenburg syndrome, a neural crest disorder, Myo10tm2/tm2 mice exhibited pigmentation defects (white belly spots) and simple syndactyly with high penetrance (>95%), and 24% of mutant embryos developed exencephalus, a neural tube closure defect. Furthermore, Myo10tm2/tm2 mice consistently displayed bilateral persistence of the hyaloid vasculature, revealed by MRI and retinal whole-mount preparations. In principle, impaired tissue clearance could contribute to persistence of hyaloid vasculature and syndactyly. However, Myo10-deficient macrophages exhibited no defects in the phagocytosis of apoptotic or IgG-opsonized cells. RNA sequence analysis showed that Myo10 was the most strongly expressed unconventional myosin in retinal vascular endothelial cells and expression levels increased 4-fold between P6 and P15, when vertical sprouting angiogenesis gives rise to deeper layers. Nevertheless, imaging of isolated adult mutant retinas did not reveal vascularization defects. In summary, Myo10 is important for both prenatal (neural tube closure and digit formation) and postnatal development (hyaloid regression, but not retinal vascularization).

Developmental dynamics of butterfly wings: real-time in vivo whole-wing imaging of twelve butterfly species

Colour pattern development of butterfly wings has been studied from several different approaches. However, developmental changes in the pupal wing tissues have rarely been documented visually. In this study, we recorded real-time developmental changes of the pupal whole wings of 9 nymphalid, 2 lycaenid, and 1 pierid species in vivo, from immediately after pupation to eclosion, using the forewing-lift method. The developmental period was roughly divided into four sequential stages. At the very early stage, the wing tissue was transparent, but at the second stage, it became semi-transparent and showed dynamic peripheral adjustment and slow low-frequency contractions. At this stage, the wing peripheral portion diminished in size, but simultaneously, the ventral epithelium expanded in size. Likely because of scale growth, the wing tissue became deeply whitish at the second and third stages, followed by pigment deposition and structural colour expression at the fourth stage. Some red or yellow (light-colour) areas that emerged early were “overpainted” by expanding black areas, suggesting the coexistence of two morphogenic signals in some scale cells. The discal spot emerged first in some nymphalid species, as though it organised the entire development of colour patterns. These results indicated the dynamic wing developmental processes common in butterflies.

Feedback regulation of cytoneme-mediated transport shapes a tissue-specific FGF morphogen gradient

Gradients of signaling proteins are essential for inducing tissue morphogenesis. However, mechanisms of gradient formation remain controversial. Here we characterized the distribution of fluorescently-tagged signaling proteins, FGF and FGFR, expressed at physiological levels from the genomic knock-in alleles in Drosophila. FGF produced in the larval wing imaginal-disc moves to the air-sac-primordium (ASP) through FGFR-containing cytonemes that extend from the ASP to contact the wing-disc source. The number of FGF-receiving cytonemes extended by ASP cells decreases gradually with increasing distance from the source, generating a recipient-specific FGF gradient. Acting as a morphogen in the ASP, FGF activates concentration-dependent gene expression, inducing pointed-P1 at higher and cut at lower levels. The transcription-factors Pointed-P1 and Cut antagonize each other and differentially regulate formation of FGFR-containing cytonemes, creating regions with higher-to-lower numbers of FGF-receiving cytonemes. These results reveal a robust mechanism where morphogens self-generate precise tissue-specific gradient contours through feedback regulation of cytoneme-mediated dispersion.

When an embryo develops, its cells must work together and ‘talk’ with each other so they can build the tissues and organs of the body. A cell can communicate with its neighbors by producing a signal, also known as a morphogen, which will tell the receiving cells what to do. Once outside the cell, a morphogen spreads through the surrounding tissue and forms a gradient: there is more of the molecule closer to the signaling cells and less further away. The cells that receive the message respond differently depending on how much morphogen they get, and therefore on where they are placed in the embryo.

How morphogens move in tissues to create gradients is still poorly understood. One hypothesis is that, once released, they spread passively through the space between cells. Instead, recent research has shown that some morphogens travel through long, thin cellular extensions known as cytonemes. These structures directly connect the cells that produce a morphogen with the ones that receive the molecule. Yet, it is still unclear how cytonemes can help to form gradients.

Du et al. aimed to resolve this question by following a morphogen called Branchless as it traveled through fruit fly embryos. Branchless is important for sculpting the embryonic airway tissue into a delicate network of branched tubes which supply oxygen to the cells of an adult fly. However, no one knew how cells communicate Branchless, whether or not Branchless formed a gradient, and if it did, how this gradient was created to set up the plan to form airway tubes. It was assumed that the molecule would diffuse passively to reach airway cells – but this is not what the experiments by Du et al. showed.

To directly observe how Branchless moves among cells, insects were genetically engineered to produce Branchless molecules attached to a fluorescent ‘tag’. Microscopy experiments using these flies revealed that Branchless did not diffuse passively; instead, airway cells used cytonemes to ‘reach’ towards the cells that produced the molecule, collecting the signal directly from its source. The gradient was created because the airway cells near the cells that make Branchless had more cytonemes, and therefore received more of the molecule compared to the cells that were placed further away. Genetic analysis of the airway tissue showed that Branchless acts as a morphogen to switch on different genes in the receiving cells placed in different locations. The target genes activated by the gradient instruct the receiving cells on how many cytonemes need to be extended, which helps the gradient to maintain itself over time.

Du et al. demonstrate for the first time how cytonemes can relay a signal to establish a gradient in a developing tissue. Dissecting how cells exchange information to create an organism could help to understand how this communication fails and leads to disorders.

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.

Direct visualization of a native Wnt in vivo reveals that a long-range Wnt gradient forms by extracellular dispersal

Wnts are evolutionarily conserved signaling proteins with essential roles in development and disease that have often been thought to spread between cells and signal at a distance. However, recent studies have challenged this model, and whether long-distance extracellular Wnt dispersal occurs and is biologically relevant is debated. Understanding fundamental aspects of Wnt dispersal has been limited by challenges with observing endogenous ligands in vivo, which has prevented directly testing hypotheses. Here, we have generated functional, fluorescently tagged alleles for a C. elegans Wnt homolog and for the first time visualized a native, long-range Wnt gradient in a living animal. Live imaging of Wnt along with source and responding cell membranes provided support for free, extracellular dispersal. By limiting Wnt transfer between cells, we confirmed that extracellular spreading shapes a long-range gradient and is critical for neuroblast migration. These results provide direct evidence that Wnts spread extracellularly to regulate aspects of long-range signaling.

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.

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.

Wnt Signaling in Stem Cell Maintenance and Differentiation in the Drosophila Germarium

Wnt signaling is a conserved regulator of stem cell behaviors, and the Drosophila germarium has been an important model tissue for the study of stem cell maintenance, differentiation, and proliferation. Here we review Wnt signaling in the germarium, which houses two distinct types of ovarian stem cells: the anteriorly located germline stem cells (GSCs), which give rise to oocytes; and the mid-posteriorly located follicle stem cells (FSCs), which give rise to the somatic follicle cells that cover a developing oocyte. The maintenance and proliferation of GSCs and FSCs is regulated by the stem cell niches, whereas differentiation of the germline is regulated by the differentiation niche. Four distinct Wnt ligands are localized in the germarium, and we focus review on how these Wnt ligands and Wnt signaling affects maintenance and differentiation of both germline and follicle stem cells in their respective niches.

Bidirectional transport model of morphogen gradient formation via cytonemes

Morphogen protein gradients play an important role in the spatial regulation of patterning during embryonic development. The most commonly accepted mechanism for gradient formation is diffusion from a source combined with degradation. Recently, there has been growing interest in an alternative mechanism, which is based on the direct delivery of morphogens along thin, actin-rich cellular extensions known as cytonemes. In this paper, we develop a bidirectional motor transport model for the flux of morphogens along cytonemes, linking a source cell to a one-dimensional array of target cells. By solving the steady-state transport equations, we show how a morphogen gradient can be established, and explore how the mean velocity of the motors affects properties of the morphogen gradient such as accumulation time and robustness. In particular, our analysis suggests that in order to achieve robustness with respect to changes in the rate of synthesis of morphogen, the mean velocity has to be negative, that is, retrograde flow or treadmilling dominates. Thus the potential targeting precision of cytonemes comes at an energy cost. We then study the effects of non-uniformly allocating morphogens to the various cytonemes projecting from a source cell. This competition for resources provides a potential regulatory control mechanism not available in diffusion-based models.