Visualization and exploration of Tcf/Lef function using a highly responsive Wnt/β-catenin signaling-reporter transgenic zebrafish

Cell proliferation and regeneration in the gill

Seminal studies from the early 20th century defined the structural changes associated with development and regeneration of the gills in goldfish at the gross morphological and cellular levels using standard techniques of light and electron microscopy. More recently, investigations using cell lineage tracing, molecular biology, immunohistochemistry and single-cell RNA-sequencing have pushed the field forward and have begun to reveal the cellular and molecular processes that orchestrate cell proliferation and regeneration in the gills. The gill is a multifunctional organ that mediates an array of important physiological functions, including respiration, ion regulation and excretion of waste products. It is comprised of unique cell types, such as pavement cells, ionocytes, chemoreceptors and undifferentiated stem or progenitor cells that regulate growth and replenish cell populations. The gills develop from the embryonic endoderm and are rich in cell types derived from the neural crest. The gills have the capacity to remodel themselves in response to environmental change, such as in the case of ionocytes, chemoreceptors and the interlamellar cell mass, and can completely regenerate gill filaments and lamellae. Both processes of remodeling and regeneration invariably involve cell proliferation. Although gill regeneration has been reported in only a limited number of fish species, the process appears to have many similarities to regeneration of other organs in fish and amphibians. The present article reviews the studies that have described gill development and growth, and that demonstrate a suite of genes, transcription factors and other proteins involved in cell proliferation and regeneration in the gills.

p65 signaling dynamics drive the developmental progression of hematopoietic stem and progenitor cells through cell cycle regulation

Most gene functions have been discovered through phenotypic observations under loss of function experiments that lack temporal control. However, cell signaling relies on limited transcriptional effectors, having to be re-used temporally and spatially within the organism. Despite that, the dynamic nature of signaling pathways have been overlooked due to the difficulty on their assessment, resulting in important bottlenecks. Here, we have utilized the rapid and synchronized developmental transitions occurring within the zebrafish embryo, in conjunction with custom NF-kB reporter embryos driving destabilized fluorophores that report signaling dynamics in real time. We reveal that NF-kB signaling works as a clock that controls the developmental progression of hematopoietic stem and progenitor cells (HSPCs) by two p65 activity waves that inhibit cell cycle. Temporal disruption of each wave results in contrasting phenotypic outcomes: loss of HSPCs due to impaired specification versus proliferative expansion and failure to delaminate from their niche. We also show functional conservation during human hematopoietic development using iPSC models. Our work identifies p65 as a previously unrecognized contributor to cell cycle regulation, revealing why and when pro-inflammatory signaling is required during HSPC development. It highlights the importance of considering and leveraging cell signaling as a temporally dynamic entity.

DHCR7 links cholesterol synthesis with neuronal development and axonal integrity

The DHCR7 enzyme converts 7-DHC into cholesterol. Mutations in DHCR7 can block cholesterol production, leading to abnormal accumulation of 7-DHC and causing Smith-Lemli-Opitz syndrome (SLOS). SLOS is an autosomal recessive disorder characterized by multiple malformations, including microcephaly, intellectual disability, behavior reminiscent of autism, sleep disturbances, and attention-deficit/hyperactivity disorder (ADHD)-like hyperactivity. Although 7-DHC affects neuronal differentiation in ex vivo experiments, the precise mechanism of SLOS remains unclear. We generated Dhcr7 deficient (dhcr7-/-) zebrafish that exhibited key features of SLOS, including microcephaly, decreased neural stem cell pools, and behavioral phenotypes similar to those of ADHD-like hyperactivity. These zebrafish demonstrated compromised myelination, synaptic anomalies, and neurotransmitter imbalances. The axons of the dhcr7-/- zebrafish showed increased lysosomes and attenuated autophagy, suggesting that autophagy-related neuronal homeostasis is disrupted.

wnt10a is required for zebrafish median fin fold maintenance and adult unpaired fin metamorphosis

BACKGROUND

Mutations of human WNT10A are associated with odonto-ectodermal dysplasia syndromes. Here, we present analyses of wnt10a loss-of-function mutants in the zebrafish.

RESULTS

wnt10a mutant zebrafish embryos display impaired tooth development and a collapsing median fin fold (MFF). Rescue experiments show that wnt10a is essential for MFF maintenance both during embryogenesis and later metamorphosis. The MFF collapse could not be attributed to increased cell death or altered proliferation rates of MFF cell types. Rather, wnt10a mutants show reduced expression levels of dlx2a in distal-most MFF cells, followed by compromised expression of col1a1a and other extracellular matrix proteins encoding genes. Transmission electron microscopy analysis shows that although dermal MFF compartments of wnt10a mutants initially are of normal morphology, with regular collagenous actinotrichia, positioning of actinotrichia within the cleft of distal MFF cells becomes compromised, coinciding with actinotrichia shrinkage and MFF collapse.

CONCLUSIONS

MFF collapse of wnt10a mutant zebrafish is likely caused by the loss of distal properties in the developing MFF, strikingly similar to the proposed molecular pathomechanisms underlying the teeth defects caused by the loss of Wnt10 in fish and mammals. In addition, it points to thus fur unknown mechanisms controlling the linear growth and stability of actinotrichia and their collagen fibrils.

Dynamics of Wnt/β-catenin reporter activity throughout whole life in a naturally short-lived vertebrate

Wnt/β-catenin signaling plays a major role in regulation of embryogenesis, organogenesis, and adult tissue homeostasis and regeneration. However, the roles played by Wnt/β-catenin and the spatiotemporal regulation of its activity throughout life, including during aging, are not fully understood. To address these issues, we introduced a Wnt/β-catenin signaling sensitive reporter into African turquoise killifish (Nothobranchius furzeri), a naturally ultra-short-lived fish that allows for the analysis of its whole life within a short period of time. Using this reporter killifish, we unraveled the previously unidentified dynamics of Wnt/β-catenin signaling during development and aging. Using the reporter strain, we detected Wnt/β-catenin activity in actively developing tissues as reported in previous reports, but also observed activation and attenuation of Wnt/β-catenin activity during embryonic reaggregation and diapause, respectively. During the aging process, the reporter was activated in the choroidal layer and liver, but its expression decreased in the kidneys. In addition, the reporter also revealed that aging disrupts the spatial regulation and intensity control of Wnt/β-catenin activity seen during fin regeneration, which interferes with precise regeneration. Thus, the employed reporter killifish is a highly useful model for investigating the dynamics of Wnt/β-catenin signaling during both the developmental and aging process.

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.

Zebrafish drug screening identifies Erlotinib as an inhibitor of Wnt/β-catenin signaling and self-renewal in T-cell acute lymphoblastic leukemia

The Wnt/β-catenin pathway's significance in cancer initiation, progression, and stem cell biology underscores its therapeutic potential. However, the clinical application of Wnt inhibitors remains limited due to challenges posed by off-target effects and complex cross-talk of Wnt signaling with other pathways. In this study, we leveraged a zebrafish model to perform a robust and rapid drug screening of 773 FDA-approved compounds to identify Wnt/β-catenin inhibitors with minimal toxicity. Utilizing zebrafish expressing a Wnt reporter, we identified several drugs that suppressed Wnt signaling without compromising zebrafish development. The efficacy of the top hit, Erlotinib, extended to human cells, where it blocked Wnt/β-catenin signaling downstream of the destruction complex. Notably, Erlotinib treatment reduced self-renewal in human T-cell Acute Lymphoblastic Leukemia cells, which rely on active β-catenin signaling for maintenance of leukemia-initiating cells. Erlotinib also reduced leukemia-initiating cell frequency and delayed disease formation in zebrafish models. This study underscores zebrafish's translational potential in drug discovery and repurposing and highlights a new use for Erlotinib as a Wnt inhibitor for cancers driven by aberrant Wnt/β-catenin signaling.

Modulation of tooth regeneration through opposing responses to Wnt and BMP signals in teleosts

Most vertebrate species undergo tooth replacement throughout adult life. This process is marked by the shedding of existing teeth and the regeneration of tooth organs. However, little is known about the genetic circuitry regulating tooth replacement. Here, we tested whether fish orthologs of genes known to regulate mammalian hair regeneration have effects on tooth replacement. Using two fish species that demonstrate distinct modes of tooth regeneration, threespine stickleback (Gasterosteus aculeatus) and zebrafish (Danio rerio), we found that transgenic overexpression of four different genes changed tooth replacement rates in the direction predicted by a hair regeneration model: Wnt10a and Grem2a increased tooth replacement rate, whereas Bmp6 and Dkk2 strongly inhibited tooth formation. Thus, similar to known roles in hair regeneration, Wnt and BMP signals promote and inhibit regeneration, respectively. Regulation of total tooth number was separable from regulation of replacement rates. RNA sequencing of stickleback dental tissue showed that Bmp6 overexpression resulted in an upregulation of Wnt inhibitors. Together, these data support a model in which different epithelial organs, such as teeth and hair, share genetic circuitry driving organ regeneration.

Cadherin-linked morphogen gradient actualizes robust tissue patterning

Morphogen gradients govern tissue patterning. These gradients provide positional information, instructing cells to adopt distinct fates. Over the past few decades, extensive studies have revealed the detailed mechanisms by which morphogens generate tissue patterns. However, the communication between morphogen-receiving cells is still poorly understood. Here, I describe how cadherin-mediated cell competition ensures robust morphogen-gradient formation. In normal zebrafish embryos, unfit cells with abnormal Wnt signaling activity spontaneously appear and produce a noisy morphogen gradient. These unfit cells communicate with neighboring cells through cadherins and are subsequently killed by cell competition. This process of killing unfit cells corrects noisy gradients to support reproducible patterning. I also discuss the significance of cell-competition-mediated morphogen-gradient correction from the perspectives of evolution and disease biology.

Determining zebrafish dorsal organizer size by a negative feedback loop between canonical/non-canonical Wnts and Tlr4/NFκB

In vertebrate embryos, the canonical Wnt ligand primes the formation of dorsal organizers that govern dorsal-ventral patterns by secreting BMP antagonists. In contrast, in Drosophila embryos, Toll-like receptor (Tlr)-mediated NFκB activation initiates dorsal-ventral patterning, wherein Wnt-mediated negative feedback regulation of Tlr/NFκB generates a BMP antagonist-secreting signalling centre to control the dorsal-ventral pattern. Although both Wnt and BMP antagonist are conserved among species, the involvement of Tlr/NFκB and feedback regulation in vertebrate organizer formation remains unclear. By imaging and genetic modification, we reveal that a negative feedback loop between canonical and non-canonical Wnts and Tlr4/NFκB determines the size of zebrafish organizer, and that Tlr/NFκB and Wnts switch initial cue and feedback mediator roles between Drosophila and zebrafish. Here, we show that canonical Wnt signalling stimulates the expression of the non-canonical Wnt5b ligand, activating the Tlr4 receptor to stimulate NFκB-mediated transcription of the Wnt antagonist frzb, restricting Wnt-dependent dorsal organizer formation.

Microglial depletion after brain injury prolongs inflammation and impairs brain repair, adult neurogenesis and pro-regenerative signaling

The adult zebrafish brain, unlike mammals, has a remarkable regenerative capacity. Although inflammation in part hinders regeneration in mammals, it is necessary for zebrafish brain repair. Microglia are resident brain immune cells that regulate the inflammatory response. To explore the microglial role in repair, we used liposomal clodronate or colony stimulating factor-1 receptor (csf1r) inhibitor to suppress microglia after brain injury, and also examined regeneration in two genetic mutant lines that lack microglia. We found that microglial ablation impaired telencephalic regeneration after injury. Microglial suppression attenuated cell proliferation at the intermediate progenitor cell amplification stage of neurogenesis. Notably, the loss of microglia impaired phospho-Stat3 (signal transducer and activator of transcription 3) and ß-Catenin signaling after injury. Furthermore, the ectopic activation of Stat3 and ß-Catenin rescued neurogenesis defects caused by microglial loss. Microglial suppression also prolonged the post-injury inflammatory phase characterized by neutrophil accumulation, likely hindering the resolution of inflammation. These findings reveal specific roles of microglia and inflammatory signaling during zebrafish telencephalic regeneration that should advance strategies to improve mammalian brain repair.

Zebrafish Drug Screening Identifies Erlotinib as an Inhibitor of Wnt/β-Catenin Signaling and Self-Renewal in T-cell Acute Lymphoblastic Leukemia

The Wnt/β-catenin pathway's significance in cancer initiation, progression, and stem cell biology underscores its therapeutic potential, yet clinical application of Wnt inhibitors remains limited due to challenges posed by off-target effects and complex crosstalk with other pathways. In this study, we leveraged the zebrafish model to perform a robust and rapid drug screening of 773 FDA-approved compounds to identify Wnt/β-catenin inhibitors with minimal toxicity. Utilizing zebrafish expressing a Wnt reporter, we identified several drugs that suppressed Wnt signaling without compromising zebrafish development. The efficacy of the top hit, Erlotinib, extended to human cells, where it blocked Wnt/β-catenin signaling downstream of the destruction complex. Notably, Erlotinib treatment reduced self-renewal in human T-cell Acute Lymphoblastic Leukemia cells, which are known to rely on active β-catenin signaling for maintenance of leukemia-initiating cells. Erlotinib also reduced leukemia-initiating cell frequency and delayed disease formation in zebrafish models. This study underscores zebrafish's translational potential in drug discovery and repurposing, and highlights a new use for Erlotinib as a Wnt inhibitor for cancers driven by aberrant Wnt/β-catenin signaling.

Highlights

Zebrafish-based drug screening offers an inexpensive and robust platform for identifying compounds with high efficacy and low toxicity in vivo . Erlotinib, an Epidermal Growth Factor Receptor (EGFR) inhibitor, emerged as a potent and promising Wnt inhibitor with effects in both zebrafish and human cell-based Wnt reporter assays.The identification of Erlotinib as a Wnt inhibitor underscores the value of repurposed drugs in developing targeted therapies to disrupt cancer stemness and improve clinical outcomes.

Genetically engineered zebrafish as models of skeletal development and regeneration

Zebrafish (Danio rerio) are aquatic vertebrates with significant homology to their terrestrial counterparts. While zebrafish have a centuries-long track record in developmental and regenerative biology, their utility has grown exponentially with the onset of modern genetics. This is exemplified in studies focused on skeletal development and repair. Herein, the numerous contributions of zebrafish to our understanding of the basic science of cartilage, bone, tendon/ligament, and other skeletal tissues are described, with a particular focus on applications to development and regeneration. We summarize the genetic strengths that have made the zebrafish a powerful model to understand skeletal biology. We also highlight the large body of existing tools and techniques available to understand skeletal development and repair in the zebrafish and introduce emerging methods that will aid in novel discoveries in skeletal biology. Finally, we review the unique contributions of zebrafish to our understanding of regeneration and highlight diverse routes of repair in different contexts of injury. We conclude that zebrafish will continue to fill a niche of increasing breadth and depth in the study of basic cellular mechanisms of skeletal biology.

Splashed E-box and AP-1 motifs cooperatively drive regeneration response and shape regeneration abilities

Injury triggers a genetic program that induces gene expression for regeneration. Recent studies have identified regeneration-response enhancers (RREs); however, it remains unclear whether a common mechanism operates in these RREs. We identified three RREs from the zebrafish fn1b promoter by searching for conserved sequences within the surrounding genomic regions of regeneration-induced genes and performed a transgenic assay for regeneration response. Two regions contained in the transposons displayed RRE activity when combined with the -0.7 kb fn1b promoter. Another non-transposon element functioned as a stand-alone enhancer in combination with a minimum promoter. By searching for transcription factor-binding motifs and validation by transgenic assays, we revealed that the cooperation of E-box and activator protein 1 motifs is necessary and sufficient for regenerative response. Such RREs respond to variety of tissue injuries, including those in the zebrafish heart and Xenopus limb buds. Our findings suggest that the fidelity of regeneration response is ensured by the two signals evoked by tissue injuries. It is speculated that a large pool of potential enhancers in the genome has helped shape the regenerative capacities during evolution.

Impact of Copper Nanoparticles and Copper Ions on Transcripts Involved in Neural Repair Mechanisms in Rainbow Trout Olfactory Mucosa

Olfactory mucosa is well known for its lifelong ability for regeneration. Regeneration of neurons and regrowth of severed axons are the most common neural repair mechanisms in olfactory mucosa. Nonetheless, exposure to neurotoxic contaminants, such as copper nanoparticles (CuNPs) and copper ions (Cu²⁺), may alter the reparative capacity of olfactory mucosa. Here, using RNA-sequencing, we investigated the molecular basis of neural repair mechanisms that were affected by CuNPs and Cu²⁺ in rainbow trout olfactory mucosa. The transcript profile of olfactory mucosa suggested that regeneration of neurons was inhibited by CuNPs. Exposure to CuNPs reduced the transcript abundances of pro-inflammatory proteins which are required to initiate neuroregeneration. Moreover, the transcript of genes encoding regeneration promoters, including canonical Wnt/β-catenin signaling proteins and developmental transcription factors, were downregulated in the CuNP-treated fish. The mRNA levels of genes regulating axonal regrowth, including the growth-promoting signals secreted from olfactory ensheathing cells, were mainly increased in the CuNP treatment. However, the reduced transcript abundances of a few cell adhesion molecules and neural polarity genes may restrict axonogenesis in the CuNP-exposed olfactory mucosa. In the Cu²⁺-treated olfactory mucosa, both neural repair strategies were initiated at the transcript level. The stimulation of repair mechanisms can lead to the recovery of Cu²⁺-induced olfactory dysfunction. These results indicated CuNPs and Cu²⁺ differentially affected the neural repair mechanism in olfactory mucosa. Exposure to CuNP had greater effects on the expression of genes involved in olfactory repair mechanisms relative to Cu²⁺ and dysregulated the transcripts associated with stem cell proliferation and neural reconstitution.

Transgenic quails reveal dynamic TCF/β-catenin signaling during avian embryonic development

The Wnt/β-catenin signaling pathway is highly conserved throughout evolution, playing crucial roles in several developmental and pathological processes. Wnt ligands can act at a considerable distance from their sources and it is therefore necessary to examine not only the Wnt-producing but also the Wnt-receiving cells and tissues to fully appreciate the many functions of this pathway. To monitor Wnt activity, multiple tools have been designed which consist of multimerized Wnt signaling response elements (TCF/LEF binding sites) driving the expression of fluorescent reporter proteins (e.g. GFP, RFP) or of LacZ. The high stability of those reporters leads to a considerable accumulation in cells activating the pathway, thereby making them easily detectable. However, this makes them unsuitable to follow temporal changes of the pathway’s activity during dynamic biological events. Even though fluorescent transcriptional reporters can be destabilized to shorten their half-lives, this dramatically reduces signal intensities, particularly when applied in vivo. To alleviate these issues, we developed two transgenic quail lines in which high copy number (12× or 16×) of the TCF/LEF binding sites drive the expression of destabilized GFP variants. Translational enhancer sequences derived from viral mRNAs were used to increase signal intensity and specificity. This resulted in transgenic lines efficient for the characterization of TCF/β-catenin transcriptional dynamic activities during embryogenesis, including using in vivo imaging. Our analyses demonstrate the use of this transcriptional reporter to unveil novel aspects of Wnt signaling, thus opening new routes of investigation into the role of this pathway during amniote embryonic development.

Macrophages Break Interneuromast Cell Quiescence by Intervening in the Inhibition of Schwann Cells in the Zebrafish Lateral Line

In the zebrafish lateral line system, interneuromast cells (INCs) between neuromasts are kept quiescent by underlying Schwann cells (SWCs). Upon severe injuries that cause the complete loss of an entire neuromast, INCs can occasionally differentiate into neuromasts but how they escape from the inhibition by SWCs is still unclear. Using a genetic/chemical method to ablate a neuromast precisely, we found that a small portion of larvae can regenerate a new neuromast. However, the residual regeneration capacity was hindered by inhibiting macrophages. Using in toto imaging, we further discovered heterogeneities in macrophage behavior and distribution along the lateral line. We witnessed the crawling of macrophages between the injured lateral line and SWCs during regeneration and between the second primordium and the first mature lateral line during development. It implies that macrophages may physically alleviate the nerve inhibition to break the dormancy of INCs during regeneration and development in the zebrafish lateral line.

Zebrafish imaging reveals TP53 mutation switching oncogene-induced senescence from suppressor to driver in primary tumorigenesis

Most tumours are thought to arise through oncogenic cell generation followed by additional mutations. How a new oncogenic cell primes tumorigenesis by acquiring additional mutations remains unclear. We show that an additional TP53 mutation stimulates primary tumorigenesis by switching oncogene-induced senescence from a tumour suppressor to a driver. Zebrafish imaging reveals that a newly emerged oncogenic cell with the Ras^G12V mutation becomes senescent and is eliminated from the epithelia, which is prevented by adding a TP53 gain-of-function mutation (TP53^R175H) into Ras^G12V cells. Surviving Ras^G12V-TP53^R175H double-mutant cells senesce and secrete senescence-associated secretory phenotype (SASP)-related inflammatory molecules that convert neighbouring normal cells into SASP factor-secreting senescent cells, generating a heterogeneous tumour-like cell mass. We identify oncogenic cell behaviours that may control the initial human tumorigenesis step. Ras and TP53 mutations and cellular senescence are frequently detected in human tumours; similar switching may occur during the initial step of human tumorigenesis.

It is unclear how a single oncogenic cell primes tumorigenesis. Here the authors visualised this behaviour using a zebrafish larval skin as a model and show that RasG12V oncogenic cell is eliminated through oncogene-senescence while a gain of function mutation in p53 alters this behaviour from tumour suppressive to tumour promoting.

Genoarchitecture of the Early Postmitotic Pretectum and the Role of Wnt Signaling in Shaping Pretectal Neurochemical Anatomy in Zebrafish

The pretectum has a distinct nuclear arrangement and complex neurochemical anatomy. While previous genoarchitectural studies have described rostrocaudal and dorsoventral progenitor domains and subdomains in different species, the relationship between these early partitions and its later derivatives in the mature anatomy is less understood. The signals and transcription factors that control the establishment of pretectal anatomy are practically unknown. We investigated the possibility that some aspects of the development of pretectal divisions are controlled by Wnt signaling, focusing on the transitional stage between neurogenesis and histogenesis in zebrafish. Using several molecular markers and following the prosomeric model, we identified derivatives from each rostrocaudal pretectal progenitor domain and described the localization of gad1b-positive GABAergic and vglut2.2-positive glutamatergic cell clusters. We also attempted to relate these clusters to pretectal nuclei in the mature brain. Then, we examined the influence of Wnt signaling on the size of neurochemically distinctive pretectal areas, using a chemical inhibitor of the Wnt pathway and the CRISPR/Cas9 approach to knock out genes that encode the Wnt pathway mediators, Lef1 and Tcf7l2. The downregulation of the Wnt pathway led to a decrease in two GABAergic clusters and an expansion of a glutamatergic subregion in the maturing pretectum. This revealed an instructive role of the Wnt signal in the development of the pretectum during neurogenesis. The molecular anatomy presented here improves our understanding of pretectal development during early postmitotic stages and support the hypothesis that Wnt signaling is involved in shaping the neurochemical organization of the pretectum.

Wnt/β-catenin signaling acts cell-autonomously to promote cardiomyocyte regeneration in the zebrafish heart

Zebrafish can achieve scar-free healing of heart injuries, and robustly replace all cardiomyocytes lost to injury via dedifferentiation and proliferation of mature cardiomyocytes. Previous studies suggested that Wnt/β-catenin signaling is active in the injured zebrafish heart, where it induces fibrosis and prevents cardiomyocyte cell cycling. Here, via targeting the destruction complex of the Wnt/β-catenin pathway with pharmacological and genetic tools, we demonstrate that Wnt/β-catenin activity is required for cardiomyocyte proliferation and dedifferentiation, as well as for maturation of the scar during regeneration. Using cardiomyocyte-specific conditional inhibition of the pathway, we show that Wnt/β-catenin signaling acts cell-autonomously to promote cardiomyocyte proliferation. Our results stand in contrast to previous reports and rather support a model in which Wnt/β-catenin signaling plays a positive role during heart regeneration in zebrafish.

Zebrafish twist2/dermo1 regulates scale shape and scale organization during skin development and regeneration

Scales are skin appendages in fishes that evolutionarily predate feathers in birds and hair in mammals. Zebrafish scales are dermal in origin and develop during metamorphosis. Understanding regulation of scale development in zebrafish offers an exciting possibility of unraveling how the mechanisms of skin appendage formation evolved in lower vertebrates and whether these mechanisms remained conserved in birds and mammals. Here we have investigated the expression and function of twist 2/dermo1 gene - known for its function in feather and hair formation - in scale development and regeneration. We show that of the four zebrafish twist paralogues, twist2/dermo1 and twist3 are expressed in the scale forming cells during scale development. Their expression is also upregulated during scale regeneration. Our knockout analysis reveals that twist2/dermo1 gene functions in the maintenance of the scale shape and organization during development as well as regeneration. We further show that the expression of twist2/dermo1 and twist3 is regulated by Wnt signaling. Our results demonstrate that the function of twist2/dermo1 in skin appendage formation, presumably under regulation of Wnt signaling, originated during evolution of basal vertebrates.

Protocol for rapid assessment of the efficacy of novel Wnt inhibitors using zebrafish models

Dysregulation of Wnt signaling is a hallmark of many cancers, and the development of effective, non-toxic small-molecule Wnt inhibitors is desirable. Off-target toxicities of new compounds are typically tested in mouse models, which is both costly and time consuming. Here, we present a rapid and inexpensive protocol to determine the in vivo toxicity and efficacy of novel Wnt inhibitors in zebrafish using a combination of a fluorescence reporter assay as well as eye rescue and fin regeneration assays. These experiments are completed within 1 week to rapidly narrow drug candidates before moving to more expensive pre-clinical testing. For complete details on the use and execution of this protocol, please refer to Zhang et al. (2020).

Attenuating the Epidermal Growth Factor Receptor-Extracellular Signal-Regulated Kinase-Sex-Determining Region Y-Box 9 Axis Promotes Liver Progenitor Cell-Mediated Liver Regeneration in Zebrafish

BACKGROUND AND AIMS

The liver is a highly regenerative organ, but its regenerative capacity is compromised in severe liver injury settings. In chronic liver diseases, the number of liver progenitor cells (LPCs) correlates proportionally to disease severity, implying that their inefficient differentiation into hepatocytes exacerbates the disease. Moreover, LPCs secrete proinflammatory cytokines; thus, their prolonged presence worsens inflammation and induces fibrosis. Promoting LPC-to-hepatocyte differentiation in patients with advanced liver disease, for whom liver transplantation is currently the only therapeutic option, may be a feasible clinical approach because such promotion generates more functional hepatocytes and concomitantly reduces inflammation and fibrosis.

APPROACH AND RESULTS

Here, using zebrafish models of LPC-mediated liver regeneration, we present a proof of principle of such therapeutics by demonstrating a role for the epidermal growth factor receptor (EGFR) signaling pathway in differentiation of LPCs into hepatocytes. We found that suppression of EGFR signaling promoted LPC-to-hepatocyte differentiation through the mitogen-activated ERK kinase (MEK)-extracellular signal-regulated kinase (ERK)-sex-determining region Y-box 9 (SOX9) cascade. Pharmacological inhibition of EGFR or MEK/ERK promoted LPC-to-hepatocyte differentiation as well as genetic suppression of the EGFR-ERK-SOX9 axis. Moreover, Sox9b overexpression in LPCs blocked their differentiation into hepatocytes. In the zebrafish liver injury model, both hepatocytes and biliary epithelial cells contributed to LPCs. EGFR inhibition promoted the differentiation of LPCs regardless of their origin. Notably, short-term treatment with EGFR inhibitors resulted in better liver recovery over the long term.

CONCLUSIONS

The EGFR-ERK-SOX9 axis suppresses LPC-to-hepatocyte differentiation during LPC-mediated liver regeneration. We suggest EGFR inhibitors as a proregenerative therapeutic drug for patients with advanced liver disease.

Distinct tooth regeneration systems deploy a conserved battery of genes

BACKGROUND

Vertebrate teeth exhibit a wide range of regenerative systems. Many species, including most mammals, reptiles, and amphibians, form replacement teeth at a histologically distinct location called the successional dental lamina, while other species do not employ such a system. Notably, a 'lamina-less' tooth replacement condition is found in a paraphyletic array of ray-finned fishes, such as stickleback, trout, cod, medaka, and bichir. Furthermore, the position, renewal potential, and latency times appear to vary drastically across different vertebrate tooth regeneration systems. The progenitor cells underlying tooth regeneration thus present highly divergent arrangements and potentials. Given the spectrum of regeneration systems present in vertebrates, it is unclear if morphologically divergent tooth regeneration systems deploy an overlapping battery of genes in their naïve dental tissues.

RESULTS

In the present work, we aimed to determine whether or not tooth progenitor epithelia could be composed of a conserved cell type between vertebrate dentitions with divergent regeneration systems. To address this question, we compared the pharyngeal tooth regeneration processes in two ray-finned fishes: zebrafish (Danio rerio) and threespine stickleback (Gasterosteus aculeatus). These two teleost species diverged approximately 250 million years ago and demonstrate some stark differences in dental morphology and regeneration. Here, we find that the naïve successional dental lamina in zebrafish expresses a battery of nine genes (bmpr1aa, bmp6, cd34, gli1, igfbp5a, lgr4, lgr6, nfatc1, and pitx2), while active Wnt signaling and Lef1 expression occur during early morphogenesis stages of tooth development. We also find that, despite the absence of a histologically distinct successional dental lamina in stickleback tooth fields, the same battery of nine genes (Bmpr1a, Bmp6, CD34, Gli1, Igfbp5a, Lgr4, Lgr6, Nfatc1, and Pitx2) are expressed in the basalmost endodermal cell layer, which is the region most closely associated with replacement tooth germs. Like zebrafish, stickleback replacement tooth germs additionally express Lef1 and exhibit active Wnt signaling. Thus, two fish systems that either have an organized successional dental lamina (zebrafish) or lack a morphologically distinct successional dental lamina (sticklebacks) deploy similar genetic programs during tooth regeneration.

CONCLUSIONS

We propose that the expression domains described here delineate a highly conserved "successional dental epithelium" (SDE). Furthermore, a set of orthologous genes is known to mark hair follicle epithelial stem cells in mice, suggesting that regenerative systems in other epithelial appendages may utilize a related epithelial progenitor cell type, despite the highly derived nature of the resulting functional organs.

Increased apoptosis, reduced Wnt/β-catenin signaling, and altered tail development in zebrafish embryos exposed to a human-relevant chemical mixture

A wide variety of anthropogenic chemicals is detected in humans and wildlife and the health effects of various chemical exposures are not well understood. Early life stages are generally the most susceptible to chemical disruption and developmental exposure can cause disease in adulthood, but the mechanistic understanding of such effects is poor. Within the EU project EDC-MixRisk, a chemical mixture (Mixture G) was identified in the Swedish pregnancy cohort SELMA by the inverse association between levels in women at around gestational week ten with birth weight of their children. This mixture was composed of mono-ethyl phthalate, mono-butyl phthalate, mono-benzyl phthalate, mono-ethylhexyl phthalate, mono-isononyl phthalate, triclosan, perfluorohexane sulfonate, perfluorooctanoic acid, and perfluorooctane sulfonate. In a series of experimental studies, we characterized effects of Mixture G on early development in zebrafish models. Here, we studied apoptosis and Wnt/β-catenin signaling which are two evolutionarily conserved signaling pathways of crucial importance during development. We determined effects on apoptosis by measuring TUNEL staining, caspase-3 activity, and acridine orange staining in wildtype zebrafish embryos, while Wnt/β-catenin signaling was assayed using a transgenic line expressing an EGFP reporter at β-catenin-regulated promoters. We found that Mixture G increased apoptosis, suppressed Wnt/β-catenin signaling in the caudal fin, and altered the shape of the caudal fin at water concentrations only 20-100 times higher than the geometric mean serum concentration in the human cohort. These findings call for awareness that pollutant mixtures like mixture G may interfere with a variety of developmental processes, possibly resulting in adverse health effects.

Mitochondrial dysfunction interferes with neural crest specification through the FoxD3 transcription factor

The neural crest is an important group of cells with pluripotency and migratory ability that is crucially involved in tissue and cell specification during development. Craniofacial shaping, sensory neurons, body asymmetry, and pigmentation are linked to neural crest functionality. Despite its prominent role in embryogenesis, neural crest specification as well as the possible part mitochondria play in such a process remains unclarified. Mitochondria are important organelles not only for respiration, but also for regulation of cell proliferation, differentiation and death. Modulation of mitochondrial fitness and depletion of mitochondrial ATP synthesis has been shown to down-regulate Wnt signaling, both in vitro and in vivo. Since Wnt signaling is one of the crucial players during neural crest induction/specification, we hypothesized a signaling cascade connecting mitochondria to embryonic development and neural crest migration and differentiation. Here, by using pharmacological and genetic modulators of mitochondrial function, we provide evidence that a crosstalk between mitochondrial energy homeostasis and Wnt signaling is important in the development of neural crest-derived tissues. Furthermore, our results highlight the possibility to modulate neural crest cell specification by tuning mitochondrial metabolism via FoxD3, an important transcription factor that is regulated by Wnt. FoxD3 ensures the correct embryonic development and contributes to the maintenance of cell stemness and to the induction of epithelial-to-mesenchymal transition. In summary, our work offers new insights into the molecular mechanism of action of FoxD3 and demonstrates that mitochondrial fitness is linked to the regulation of this important transcription factor via Wnt signaling in the context of neural crest specification.

Suppression of Inflammation Delays Hair Cell Regeneration and Functional Recovery Following Lateral Line Damage in Zebrafish Larvae

Cochlear hair cells in human beings cannot regenerate after loss; however, those in fish and other lower species can. Recently, the role of inflammation in hair cell regeneration has been attracting the attention of scientists. In the present study, we investigated how suppression of inflammatory factors affects hair cell regeneration and the functional recovery of regenerated hair cells in zebrafish. We killed hair cells in the lateral line of zebrafish larvae with CuSO₄ to induce an inflammatory response and coapplied BRS-28, an anti-inflammatory agent to suppress the inflammation. The recovery of the hair cell number and rheotaxis was slower when CuSO₄ and BRS-28 were coapplied than when CuSO₄ was applied alone. The recovery of hair cell count lagged behind that of the calcium imaging signal during the regeneration. The calcium imaging signal in the neuromasts in the inflammation-inhibited group was weaker than that in the noninflammation-inhibited group at the early stage of regeneration, although it returned to normal at the late stage. Our study demonstrates that suppressing inflammation by BRS-28 delays hair cell regeneration and functional recovery when hair cells are damaged. We suspect that BRS-28 inhibits pro-inflammatory factors and thereby reduces the migration of macrophages to delay the regeneration of hair cells.

Comparative Transcriptome Analysis of the Regenerating Zebrafish Telencephalon Unravels a Resource With Key Pathways During Two Early Stages and Activation of Wnt/β-Catenin Signaling at the Early Wound Healing Stage

Owing to its pronounced regenerative capacity in many tissues and organs, the zebrafish brain represents an ideal platform to understand the endogenous regeneration mechanisms that restore tissue integrity and function upon injury or disease. Although radial glial and neuronal cell populations have been characterized with respect to specific marker genes, comprehensive transcriptomic profiling of the regenerating telencephalon has not been conducted so far. Here, by processing the lesioned and unlesioned hemispheres of the telencephalon separately, we reveal the differentially expressed genes (DEGs) at the early wound healing and early proliferative stages of regeneration, i.e., 20 h post-lesion (hpl) and 3 days post-lesion (dpl), respectively. At 20 hpl, we detect a far higher number of DEGs in the lesioned hemisphere than in the unlesioned half and only 7% of all DEGs in both halves. However, this difference disappears at 3 dpl, where the lesioned and unlesioned hemispheres share 40% of all DEGs. By performing an extensive comparison of the gene expression profiles in these stages, we unravel that the lesioned hemispheres at 20 hpl and 3 dpl exhibit distinct transcriptional profiles. We further unveil a prominent activation of Wnt/β-catenin signaling at 20 hpl, returning to control level in the lesioned site at 3 dpl. Wnt/β-catenin signaling indeed appears to control a large number of genes associated primarily with the p53, apoptosis, forkhead box O (FoxO), mitogen-activated protein kinase (MAPK), and mammalian target of rapamycin (mTOR) signaling pathways specifically at 20 hpl. Based on these results, we propose that the lesioned and unlesioned hemispheres react to injury dynamically during telencephalon regeneration and that the activation of Wnt/β-catenin signaling at the early wound healing stage plays a key role in the regulation of cellular and molecular events.

A phospho-switch controls RNF43-mediated degradation of Wnt receptors to suppress tumorigenesis

Frequent mutation of the tumour suppressor RNF43 is observed in many cancers, particularly colon malignancies. RNF43, an E3 ubiquitin ligase, negatively regulates Wnt signalling by inducing degradation of the Wnt receptor Frizzled. In this study, we discover that RNF43 activity requires phosphorylation at a triplet of conserved serines. This phospho-regulation of RNF43 is required for zebrafish development and growth of mouse intestinal organoids. Cancer-associated mutations that abrogate RNF43 phosphorylation cooperate with active Ras to promote tumorigenesis by abolishing the inhibitory function of RNF43 in Wnt signalling while maintaining its inhibitory function in p53 signalling. Our data suggest that RNF43 mutations cooperate with KRAS mutations to promote multi-step tumorigenesis via the Wnt-Ras-p53 axis in human colon cancers. Lastly, phosphomimetic substitutions of the serine trio restored the tumour suppressive activity of extracellular oncogenic mutants. Therefore, harnessing phospho-regulation of RNF43 might be a potential therapeutic strategy for tumours with RNF43 mutations.

Tendon Cell Regeneration Is Mediated by Attachment Site-Resident Progenitors and BMP Signaling

The musculoskeletal system is a striking example of how cell identity and position is coordinated across multiple tissues to ensure function. However, it is unclear upon tissue loss, such as complete loss of cells of a central musculoskeletal connecting tendon, whether neighboring tissues harbor progenitors capable of mediating regeneration. Here, using a zebrafish model, we genetically ablate all embryonic tendon cells and find complete regeneration of tendon structure and pattern. We identify two regenerative progenitor populations, sox10+ perichondrial cells surrounding cartilage and nkx2.5+ cells surrounding muscle. Surprisingly, laser ablation of sox10+ cells, but not nkx2.5+ cells, increases tendon progenitor number in the perichondrium, suggesting a mechanism to regulate attachment location. We find BMP signaling is active in regenerating progenitor cells and is necessary and sufficient for generating new scxa+ cells. Our work shows that muscle and cartilage connective tissues harbor progenitor cells capable of fully regenerating tendons, and this process is regulated by BMP signaling.

In Situ Fucosylation of the Wnt Co-receptor LRP6 Increases Its Endocytosis and Reduces Wnt/β-Catenin Signaling

Wnt/β-catenin signaling regulates critical, context-dependent transcription in numerous physiological events. Among the well-documented mechanisms affecting Wnt/β-catenin activity, modification of N-glycans by L-fucose is the newest and the least understood. Using a combination of Chinese hamster ovary cell mutants with different fucosylation levels and cell-surface fucose editing (in situ fucosylation [ISF]), we report that α(1-3)-fucosylation of N-acetylglucosamine (GlcNAc) in the Galβ(1-4)-GlcNAc sequences of complex N-glycans modulates Wnt/β-catenin activity by regulating the endocytosis of low-density lipoprotein receptor-related protein 6 (LRP6). Pulse-chase experiments reveal that ISF elevates endocytosis of lipid-raft-localized LRP6, leading to the suppression of Wnt/β-catenin signaling. Remarkably, Wnt activity decreased by ISF is fully reversed by the exogenously added fucose. The combined data show that in situ cell-surface fucosylation can be exploited to regulate a specific signaling pathway via endocytosis promoted by a fucose-binding protein, thereby linking glycosylation of a receptor with its intracellular signaling.

The mevalonate pathway is a crucial regulator of tendon cell specification

Tendons and ligaments are crucial components of the musculoskeletal system, yet the pathways specifying these fates remain poorly defined. Through a screen of known bioactive chemicals in zebrafish, we identified a new pathway regulating tendon cell induction. We established that statin, through inhibition of the mevalonate pathway, causes an expansion of the tendon progenitor population. Co-expression and live imaging studies indicate that the expansion does not involve an increase in cell proliferation, but rather results from re-specification of cells from the neural crest-derived sox9a⁺/sox10⁺ skeletal lineage. The effect on tendon cell expansion is specific to the geranylgeranylation branch of the mevalonate pathway and is mediated by inhibition of Rac activity. This work establishes a novel role for the mevalonate pathway and Rac activity in regulating specification of the tendon lineage.

Horizontal Boundary Cells, a Special Group of Somitic Cells, Play Crucial Roles in the Formation of Dorsoventral Compartments in Teleost Somite

Establishment of robust gene expression boundary is crucial for creating elaborate morphology during development. However, mechanisms underlying boundary formation have been extensively studied only in a few model systems. We examined the establishment of zic1/zic4-expression boundary demarcating dorsoventral boundary of the entire trunk of medaka fish (Oryzias latipes) and identified a subgroup of dermomyotomal cells called horizontal boundary cells (HBCs) as crucial players for the boundary formation. Embryological and genetic analyses demonstrated that HBCs play crucial roles in the two major events of the process, i.e., refinement and maintenance. In the refinement, HBCs could serve as a chemical barrier against Wnts from the neural tube by expressing Hhip. At later stages, HBCs participate in the maintenance of the boundary by differentiating into the horizontal myoseptum physically inhibiting cell mixing across the boundary. These findings reveal the mechanisms underlying the dorsoventral boundary in the teleost trunk by specialized boundary cells.

Wnt/β-catenin signaling is an evolutionarily conserved determinant of chordate dorsal organizer

Deciphering the mechanisms of axis formation in amphioxus is a key step to understanding the evolution of chordate body plan. The current view is that Nodal signaling is the only factor promoting the dorsal axis specification in the amphioxus, whereas Wnt/β-catenin signaling plays no role in this process. Here, we re-examined the role of Wnt/βcatenin signaling in the dorsal/ventral patterning of amphioxus embryo. We demonstrated that the spatial activity of Wnt/β-catenin signaling is located in presumptive dorsal cells from cleavage to gastrula stage, and provided functional evidence that Wnt/β-catenin signaling is necessary for the specification of dorsal cell fate in a stage-dependent manner. Microinjection of Wnt8 and Wnt11 mRNA induced ectopic dorsal axis in neurulae and larvae. Finally, we demonstrated that Nodal and Wnt/β-catenin signaling cooperate to promote the dorsal-specific gene expression in amphioxus gastrula. Our study reveals high evolutionary conservation of dorsal organizer formation in the chordate lineage.

Rab5c-mediated endocytic trafficking regulates hematopoietic stem and progenitor cell development via Notch and AKT signaling

It is well known that various developmental signals play diverse roles in hematopoietic stem and progenitor cell (HSPC) production; however, how these signaling pathways are orchestrated remains incompletely understood. Here, we report that Rab5c is essential for HSPC specification by endocytic trafficking of Notch and AKT signaling in zebrafish embryos. Rab5c deficiency leads to defects in HSPC production. Mechanistically, Rab5c regulates hemogenic endothelium (HE) specification by endocytic trafficking of Notch ligands and receptor. We further show that the interaction between Rab5c and Appl1 in the endosome is required for the survival of HE in the ventral wall of the dorsal aorta through AKT signaling. Interestingly, Rab5c overactivation can also lead to defects in HSPC production, which is attributed to excessive endolysosomal trafficking inducing Notch signaling defect. Taken together, our findings establish a previously unrecognized role of Rab5c-mediated endocytic trafficking in HSPC development and provide new insights into how spatiotemporal signals are orchestrated to accurately execute cell fate transition.

Cell-autonomous Notch signaling regulated by the membrane trafficking protein Rab5c plays an instructive role in hematopoietic stem and progenitor cell specification, while the AKT signaling seems to provide a permissive signal to maintain hemogenic endothelium survival.

Cell competition corrects noisy Wnt morphogen gradients to achieve robust patterning in the zebrafish embryo

Morphogen signalling forms an activity gradient and instructs cell identities in a signalling strength-dependent manner to pattern developing tissues. However, developing tissues also undergo dynamic morphogenesis, which may produce cells with unfit morphogen signalling and consequent noisy morphogen gradients. Here we show that a cell competition-related system corrects such noisy morphogen gradients. Zebrafish imaging analyses of the Wnt/β-catenin signalling gradient, which acts as a morphogen to establish embryonic anterior-posterior patterning, identify that unfit cells with abnormal Wnt/β-catenin activity spontaneously appear and produce noise in the gradient. Communication between unfit and neighbouring fit cells via cadherin proteins stimulates apoptosis of the unfit cells by activating Smad signalling and reactive oxygen species production. This unfit cell elimination is required for proper Wnt/β-catenin gradient formation and consequent anterior-posterior patterning. Because this gradient controls patterning not only in the embryo but also in adult tissues, this system may support tissue robustness and disease prevention.

Gradients of morphogens such as Wnt provide instructive cues for cell identities during development. Here, the authors report that in the developing zebrafish embryo, cell competition and elimination of unfit cells are required for proper Wnt gradient formation.

Integrating yeast chemical genomics and mammalian cell pathway analysis

Chemical genomics has been applied extensively to evaluate small molecules that modulate biological processes in Saccharomyces cerevisiae. Here, we use yeast as a surrogate system for studying compounds that are active against metazoan targets. Large-scale chemical-genetic profiling of thousands of synthetic and natural compounds from the Chinese National Compound Library identified those with high-confidence bioprocess target predictions. To discover compounds that have the potential to function like therapeutic agents with known targets, we also analyzed a reference library of approved drugs. Previously uncharacterized compounds with chemical-genetic profiles resembling existing drugs that modulate autophagy and Wnt/β-catenin signal transduction were further examined in mammalian cells, and new modulators with specific modes of action were validated. This analysis exploits yeast as a general platform for predicting compound bioactivity in mammalian cells.

Somatic Mutations of lats2 Cause Peripheral Nerve Sheath Tumors in Zebrafish

The cellular signaling pathways underlying peripheral nerve sheath tumor (PNST) formation are poorly understood. Hippo signaling has been recently implicated in the biology of various cancers, and is thought to function downstream of mutations in the known PNST driver, NF2. Utilizing CRISPR-Cas9 gene editing, we targeted the canonical Hippo signaling kinase Lats2. We show that, while germline deletion leads to early lethality, targeted somatic mutations of zebrafish lats2 leads to peripheral nerve sheath tumor formation. These peripheral nerve sheath tumors exhibit high levels of Hippo effectors Yap and Taz, suggesting that dysregulation of these transcriptional co-factors drives PNST formation in this model. These data indicate that somatic lats2 deletion in zebrafish can serve as a powerful experimental platform to probe the mechanisms of PNST formation and progression.

Impaired Mitochondrial ATP Production Downregulates Wnt Signaling via ER Stress Induction

Wnt signaling affects fundamental development pathways and, if aberrantly activated, promotes the development of cancers. Wnt signaling is modulated by different factors, but whether the mitochondrial energetic state affects Wnt signaling is unknown. Here, we show that sublethal concentrations of different compounds that decrease mitochondrial ATP production specifically downregulate Wnt/β-catenin signaling in vitro in colon cancer cells and in vivo in zebrafish reporter lines. Accordingly, fibroblasts from a GRACILE syndrome patient and a generated zebrafish model lead to reduced Wnt signaling. We identify a mitochondria-Wnt signaling axis whereby a decrease in mitochondrial ATP reduces calcium uptake into the endoplasmic reticulum (ER), leading to endoplasmic reticulum stress and to impaired Wnt signaling. In turn, the recovery of the ATP level or the inhibition of endoplasmic reticulum stress restores Wnt activity. These findings reveal a mechanism that links mitochondrial energetic metabolism to the control of the Wnt pathway that may be beneficial against several pathologies.

Wnt/β-catenin signaling in brain development and mental disorders: keeping TCF7L2 in mind

Canonical Wnt signaling, which is transduced by β-catenin and lymphoid enhancer factor 1/T cell-specific transcription factors (LEF1/TCFs), regulates many aspects of metazoan development and tissue renewal. Although much evidence has associated canonical Wnt/β-catenin signaling with mood disorders, the mechanistic links are still unknown. Many components of the canonical Wnt pathway are involved in cellular processes that are unrelated to classical canonical Wnt signaling, thus further blurring the picture. The present review critically evaluates the involvement of classical Wnt/β-catenin signaling in developmental processes that putatively underlie the pathology of mental illnesses. Particular attention is given to the roles of LEF1/TCFs, which have been discussed surprisingly rarely in this context. Highlighting recent discoveries, we propose that alterations in the activity of LEF1/TCFs, and particularly of transcription factor 7-like 2 (TCF7L2), result in defects previously associated with neuropsychiatric disorders, including imbalances in neurogenesis and oligodendrogenesis, the functional disruption of thalamocortical circuitry and dysfunction of the habenula.

TRAF6 function as a novel co-regulator of Wnt3a target genes in prostate cancer

Background

Tumour necrosis factor receptor associated factor 6 (TRAF6) promotes inflammation in response to various cytokines. Aberrant Wnt3a signals promotes cancer progression through accumulation of β-Catenin. Here we investigated a potential role for TRAF6 in Wnt signaling.

Methods

TRAF6 expression was silenced by siRNA in human prostate cancer (PC3U) and human colorectal SW480 cells and by CRISPR/Cas9 in zebrafish. Several biochemical methods and analyses of mutant phenotype in zebrafish were used to analyse the function of TRAF6 in Wnt signaling.

Findings

Wnt3a-treatment promoted binding of TRAF6 to the Wnt co-receptors LRP5/LRP6 in PC3U and LNCaP cells in vitro. TRAF6 positively regulated mRNA expression of β-Catenin and subsequent activation of Wnt target genes in PC3U cells. Wnt3a-induced invasion of PC3U and SW480 cells were significantly reduced when TRAF6 was silenced by siRNA. Database analysis revealed a correlation between TRAF6 mRNA and Wnt target genes in patients with prostate cancer, and high expression of LRP5, TRAF6 and c-Myc correlated with poor prognosis. By using CRISPR/Cas9 to silence TRAF6 in zebrafish, we confirm TRAF6 as a key molecule in Wnt3a signaling for expression of Wnt target genes.

Interpretation

We identify TRAF6 as an important component in Wnt3a signaling to promote activation of Wnt target genes, a finding important for understanding mechanisms driving prostate cancer progression.

Fund

KAW 2012.0090, CAN 2017/544, Swedish Medical Research Council (2016-02513), Prostatacancerförbundet, Konung Gustaf V:s Frimurarestiftelse and Cancerforskningsfonden Norrland. The funders did not play a role in manuscript design, data collection, data analysis, interpretation nor writing of the manuscript.

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TRAF6 positively regulated mRNA expression of b-Catenin and subsequent activation of Wnt target genes in prostate cancer cells in vitro.

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High expression of LRP5, TRAF6 and c-Myc correlated with poor prognosis for patients with prostate cancer.

A tannin compound from Sanguisorba officinalis blocks Wnt/β-catenin signaling pathway and induces apoptosis of colorectal cancer cells

BACKGROUND

Sanguisorba officinalis, a popular Chinese herb, called DiYu, has been shown to inhibit the growth of many human cancer cell lines, including colorectal cancer cells. The aims of this study were to discover the active compound and molecular mechanism of S. officinalis against Wnt/β-catenin signaling pathway and develop Wnt inhibitors from natural products as anti-colorectal cancer agents.

METHODS

1,4,6-Tri-O-galloyl-β-d-glucopyranose (TGG) was obtained by the preparative HPLC. The effect of DiYu on proliferation of NIH3T3 and HT29 was detected by MTT assay. Luciferase reporter assay was applied to investigate the activity of Wnt/β-catenin signaling in NIH3T3. The expression levels of mRNA and protein were detected by RT-PCR and western blot. Immunofluorescence assay was used to measure the level of β-catenin in cytoplasm and nucleus. Transcriptomic profiling study was performed to investigate the molecular mechanism of DiYu on the Wnt/β-catenin signaling pathway.

RESULTS

TGG significantly inhibited the Wnt/β-catenin signaling pathway, down-regulated the expression of β-catenin and Wnt target genes (Dkk1, c-Myc, FGF20, NKD1, Survivin), up-regulated the levels of cleaved caspase3, cleaved PARP and ratio of Bax/Bcl-2, which may explain the apoptosis of HT29.

CONCLUSIONS

Our study enhanced the discovery of the materials and elucidation of mechanisms that account for the anti-Wnt activity of natural inhibitor (DiYu) and identified the potential of TGG to be developed as anti-colorectal cancer drugs.

Wnt signaling mediates new nephron formation during zebrafish kidney regeneration

Zebrafish kidneys use resident kidney stem cells to replace damaged tubules with new nephrons: the filtration units of the kidney. What stimulates kidney progenitor cells to form new nephrons is not known. Here, we show that wnt9a and wnt9b are induced in the injured kidney at sites where frizzled9b- and lef1-expressing progenitor cells form new nephrons. New nephron aggregates are patterned by Wnt signaling, with high canonical Wnt-signaling cells forming a single cell thick rosette that demarcates: domains of cell proliferation in the elongating nephron; and tubule fusion where the new nephron plumbs into the distal tubule and establishes blood filtrate drainage. Pharmacological blockade of canonical Wnt signaling inhibited new nephron formation after injury by inhibiting cell proliferation, and resulted in loss of polarized rosette structures in the aggregates. Mutation in frizzled9b reduced total kidney nephron number, caused defects in tubule morphology and reduced regeneration of new nephrons after injury. Our results demonstrate an essential role for Wnt/frizzled signaling in adult zebrafish kidney development and regeneration, highlighting conserved mechanisms underlying both mammalian kidney development and kidney stem cell-directed neonephrogenesis in zebrafish.

Summary: Adult zebrafish kidneys induce Wnt signaling to generate new nephrons from resident kidney progenitor cells, highlighting how embryonic morphogens are reactivated in adult organs to drive regeneration.

Temporal Control of WNT Activity Regulates Tooth Number in Fish

Wnts determine cell polarity, cell proliferation, and cell differentiation during embryogenesis and play an essential role during tooth development initiation and morphogenesis. Wnt/β-catenin signaling has a time-dependent role in development because various signaling molecules that mutually interact are involved in the pathway, and tight regulation of the pathway is essential for normal development. Studies investigating how the Wnt/β-catenin signaling pathway controls the different stages of tooth development are rare. Specifically, the effects of Wnt/β-catenin signaling loss of function on different stages of tooth development are currently unknown. Here, we report the stage-dependent role of Wnt/β-catenin signaling in tooth development. In vivo loss and gain of function of Wnt/β-catenin signaling were implemented through the genetic overexpression of DKK1 with heat shock-inducible transgenic models and the pharmacologic inhibition of β-catenin destruction complex formation in zebrafish, respectively. We demonstrated that transient inhibition of Wnt/β-catenin signaling interrupted tooth development in a stage-dependent manner and conditional activation of Wnt/β-catenin signaling during 4V morphogenesis inhibited the development of 3V. These findings suggest that Wnt/β-catenin signaling plays an important role in the morphogenesis of teeth and the initiation of sequential tooth development in a stage-dependent manner.

Fsh stimulates Leydig cell Wnt5a production, enriching zebrafish type A spermatogonia

Follicle-stimulating hormone (Fsh) modulates vertebrate spermatogenesis by regulating somatic cell functions in the testis. We have found previously that zebrafish Fsh stimulated the differentiating proliferation of type A undifferentiated spermatogonia (Aund) in an androgen-independent manner by regulating the production of growth factors and other signaling molecules in both Sertoli (SCs) and Leydig cells (LCs). For example, Fsh triggered the release of Igf3 that subsequently activated β-catenin signaling to promote the differentiating proliferation of Aund. In the present study, we report that Fsh moreover uses the non-canonical Wnt pathway to promote the proliferation and accumulation of Aund. Initially, we found that the stimulatory effect of Fsh on the proliferation activity of Aund was further strengthened when β-catenin signaling was inhibited, resulting in an accumulation of Aund. We then showed that this Fsh-induced accumulation of Aund was associated with increased transcript levels of the non-canonical Wnt ligand, wnt5a. In situ hybridization of insl3 mRNA, a gene expressed in LCs, combined with Wnt5a immunocytochemistry identified LCs as the cellular source of Wnt5a in the adult zebrafish testis. Addition of an antagonist of Wnt5a to incubations with Fsh decreased both the proliferation activity and the relative section area occupied by Aund, while an agonist of Wnt5a increased these same parameters for Aund. Taken together, our data suggest that Fsh triggered LCs to release Wnt5a, which then promoted the proliferation and accumulation of Aund. Hence, Fsh uses non-canonical Wnt signaling to ensure the production of Aund, while also triggering β-catenin signaling via Igf3 to ensure spermatogonial differentiation.

Dynamic control of proinflammatory cytokines Il-1β and Tnf-α by macrophages in zebrafish spinal cord regeneration

Spinal cord injury leads to a massive response of innate immune cells in non-regenerating mammals, but also in successfully regenerating zebrafish. However, the role of the immune response in successful regeneration is poorly defined. Here we show that inhibiting inflammation reduces and promoting it accelerates axonal regeneration in spinal-lesioned zebrafish larvae. Mutant analyses show that peripheral macrophages, but not neutrophils or microglia, are necessary for repair. Macrophage-less irf8 mutants show prolonged inflammation with elevated levels of Tnf-α and Il-1β. Inhibiting Tnf-α does not rescue axonal growth in irf8 mutants, but impairs it in wildtype animals, indicating a pro-regenerative role of Tnf-α. In contrast, decreasing Il-1β levels or number of Il-1β⁺ neutrophils rescue functional regeneration in irf8 mutants. However, during early regeneration, interference with Il-1β function impairs regeneration in irf8 and wildtype animals. Hence, inflammation is dynamically controlled by macrophages to promote functional spinal cord regeneration in zebrafish.

While proinflammatory signalling is preventive to axon regrowth, activated macrophages can be beneficial, for example by limiting the inflammation. This study uses mutant zebrafish lines that lack macrophages and/or microglia to show that peripheral macrophages are necessary in axon regrowth following complete transection of spinal cord.

Igf3 activates β-catenin signaling to stimulate spermatogonial differentiation in zebrafish

Follicle-stimulating hormone (Fsh) is a major regulator of spermatogenesis, targeting somatic cell functions in the testes. We reported previously that zebrafish Fsh promoted the differentiation of type A undifferentiated spermatogonia (A_und) by stimulating the production of factors that advance germ cell differentiation, such as androgens, insulin-like peptide 3 (Insl3) and insulin-like growth factor 3 (Igf3). In addition, Fsh also modulated the transcript levels of several other genes, including some belonging to the Wnt signaling pathway. Here, we evaluated if and how Fsh utilizes part of the canonical Wnt pathway to regulate the development of spermatogonia. We quantified the proliferation activity and relative section areas occupied by A_und and type A differentiating (A_diff) spermatogonia and we analyzed the expression of selected genes in response to recombinant proteins and pharmacological inhibitors. We found that from the three downstream mediators of Fsh activity we examined, Igf3, but not 11-ketotestosterone or Insl3, modulated the transcript levels of two β-catenin sensitive genes (cyclinD1 and axin2). Using a zebrafish β-catenin signaling reporter line, we showed that Igf3 activated β-catenin signaling in type A spermatogonia and that this activation did not depend on the release of Wnt ligands. Pharmacological inhibition of the β-catenin or of the phosphoinositide 3-kinase (PI3K) pathways revealed that Igf3 activated β-catenin signaling in a manner involving PI3K to promote the differentiation of A_und to A_diff spermatogonia. This mechanism represents an intriguing example for a pituitary hormone like Fsh using Igf signaling to recruit the evolutionary conserved, local β-catenin signaling pathway to regulate spermatogenesis.

Anosmin1 Shuttles Fgf to Facilitate Its Diffusion, Increase Its Local Concentration, and Induce Sensory Organs

Growth factors induce and pattern sensory organs, but how their distribution is regulated by the extracellular matrix (ECM) is largely unclear. To address this question, we analyzed the diffusion behavior of Fgf10 molecules during sensory organ formation in the zebrafish posterior lateral line primordium. In this tissue, secreted Fgf10 induces organ formation at a distance from its source. We find that most Fgf10 molecules are highly diffusive and move rapidly through the ECM. We identify Anosmin1, which when mutated in humans causes Kallmann Syndrome, as an ECM protein that binds to Fgf10 and facilitates its diffusivity by increasing the pool of fast-moving Fgf10 molecules. In the absence of Anosmin1, Fgf10 levels are reduced and organ formation is impaired. Global overexpression of Anosmin1 slows the fast-moving Fgf10 molecules and results in Fgf10 dispersal. These results suggest that Anosmin1 liberates ECM-bound Fgf10 and shuttles it to increase its signaling range.

Retinal Wnt signaling defect in a zebrafish fetal alcohol spectrum disorder model

Fetal alcohol spectrum disorder caused by prenatal alcohol exposure includes ocular abnormalities (microphthalmia, photoreceptor dysfunction, cataracts). Zebrafish embryos exposed to ethanol from gastrulation through somitogenesis show severe ocular defects, including microphthalmia and photoreceptor differentiation defects. Ethanol-treated zebrafish had an enlarged ciliary marginal zone (CMZ) relative to the retina size and reduced Müller glial cells (MGCs). Ethanol exposure produced immature photoreceptors with increased proliferation, indicating cell cycle exit failure. Signaling mechanisms in the CMZ were affected by embryonic ethanol exposure, including Wnt signaling in the CMZ, Notch signaling and neurod gene expression. Retinoic acid or folic acid co-supplementation with ethanol rescued Wnt signaling and retinal differentiation. Activating Wnt signaling using GSK3 inhibitor (LSN 2105786; Eli Lilly and Co.) restored retinal cell differentiation pathways. Ethanol exposed embryos were treated with Wnt agonist, which rescued Wnt-active cells in the CMZ, Notch-active cells in the retina, proliferation, and photoreceptor terminal differentiation. Our results illustrate the critical role of Wnt signaling in ethanol-induced retinal defects.