Distinct Wnt signaling pathways have opposing roles in appendage regeneration

Regeneration of the zebrafish retinal pigment epithelium after widespread genetic ablation

The retinal pigment epithelium (RPE) is a specialized monolayer of pigmented cells within the eye that is critical for maintaining visual system function. Diseases affecting the RPE have dire consequences for vision, and the most prevalent of these is atrophic (dry) age-related macular degeneration (AMD), which is thought to result from RPE dysfunction and degeneration. An intriguing possibility for treating RPE degenerative diseases like atrophic AMD is the stimulation of endogenous RPE regeneration; however, very little is known about the mechanisms driving successful RPE regeneration in vivo. Here, we developed a zebrafish transgenic model (rpe65a:nfsB-eGFP) that enabled ablation of large swathes of mature RPE. RPE ablation resulted in rapid RPE degeneration, as well as degeneration of Bruch’s membrane and underlying photoreceptors. Using this model, we demonstrate for the first time that zebrafish are capable of regenerating a functional RPE monolayer after RPE ablation. Regenerated RPE cells first appear at the periphery of the RPE, and regeneration proceeds in a peripheral-to-central fashion. RPE ablation elicits a robust proliferative response in the remaining RPE. Subsequently, proliferative cells move into the injury site and differentiate into RPE. BrdU incorporation assays demonstrate that the regenerated RPE is likely derived from remaining peripheral RPE cells. Pharmacological disruption using IWR-1, a Wnt signaling antagonist, significantly reduces cell proliferation in the RPE and impairs overall RPE recovery. These data demonstrate that the zebrafish RPE possesses a robust capacity for regeneration and highlight a potential mechanism through which endogenous RPE regenerate in vivo.

Diseases resulting in retinal pigment epithelium (RPE) degeneration are among the leading causes of blindness worldwide, and no therapy exists that can replace RPE or restore lost vision. One intriguing possibility is the development of therapies focused on stimulating endogenous RPE regeneration. For this to be possible, we must first gain a deeper understanding of the mechanisms underlying RPE regeneration. Here, we develop a transgenic zebrafish system through which we ablate large swathes of mature RPE and demonstrate that zebrafish regenerate RPE after widespread injury. Injury-adjacent RPE proliferate and regenerate RPE, suggesting that they are the source of regenerated tissue. Finally, we demonstrate that Wnt signaling may be involved in RPE regeneration. These findings establish a versatile in vivo model through which the molecular and cellular underpinnings of RPE regeneration can be further characterized.

scRNA-Seq reveals distinct stem cell populations that drive hair cell regeneration after loss of Fgf and Notch signaling

Loss of sensory hair cells leads to deafness and balance deficiencies. In contrast to mammalian hair cells, zebrafish ear and lateral line hair cells regenerate from poorly characterized support cells. Equally ill-defined is the gene regulatory network underlying the progression of support cells to differentiated hair cells. scRNA-Seq of lateral line organs uncovered five different support cell types, including quiescent and activated stem cells. Ordering of support cells along a developmental trajectory identified self-renewing cells and genes required for hair cell differentiation. scRNA-Seq analyses of fgf3 mutants, in which hair cell regeneration is increased, demonstrates that Fgf and Notch signaling inhibit proliferation of support cells in parallel by inhibiting Wnt signaling. Our scRNA-Seq analyses set the foundation for mechanistic studies of sensory organ regeneration and is crucial for identifying factors to trigger hair cell production in mammals. The data is searchable and publicly accessible via a web-based interface.

Blastemal progenitors modulate immune signaling during early limb regeneration

Blastema formation, a hallmark of limb regeneration, requires proliferation and migration of progenitors to the amputation plane. Although blastema formation has been well described, the transcriptional programs that drive blastemal progenitors remain unknown. We transcriptionally profiled dividing and non-dividing cells in regenerating stump tissues, as well as the wound epidermis, during early axolotl limb regeneration. Our analysis revealed unique transcriptional signatures of early dividing cells and, unexpectedly, repression of several core developmental signaling pathways in early regenerating stump tissues. We further identify an immunomodulatory role for blastemal progenitors through interleukin 8 (IL-8), a highly expressed cytokine in subpopulations of early blastemal progenitors. Ectopic il-8 expression in non-regenerating limbs induced myeloid cell recruitment, while IL-8 knockdown resulted in defective myeloid cell retention during late wound healing, delaying regeneration. Furthermore, the il-8 receptor cxcr-1/2 was expressed in myeloid cells, and inhibition of CXCR-1/2 signaling during early stages of limb regeneration prevented regeneration. Altogether, our findings suggest that blastemal progenitors are active early mediators of immune support, and identify CXCR-1/2 signaling as an important immunomodulatory pathway during the initiation of regeneration.

Evolution and Regulation of Limb Regeneration in Arthropods

Regeneration has fascinated both scientists and non-scientists for centuries. Many organisms can regenerate, and arthropod limbs are no exception although their ability to regenerate is a product shaped by natural and sexual selection. Recent studies have begun to uncover cellular and molecular processes underlying limb regeneration in several arthropod species. Here we argue that an evo-devo approach to the study of arthropod limb regeneration is needed to understand aspects of limb regeneration that are conserved and divergent. In particular, we argue that limbs of different species are comprised of cells at distinct stages of differentiation at the time of limb loss and therefore provide insights into regeneration involving both stem cell-like cells/precursor cells and differentiated cells. In addition, we review recent studies that demonstrate how limb regeneration impacts the development of the whole organism and argue that studies on the link between local tissue damage and the rest of the body should provide insights into the integrative nature of development. Molecular studies on limb regeneration are only beginning to take off, but comparative studies on the mechanisms of limb regeneration across various taxa should not only yield interesting insights into development but also answer how this remarkable ability evolved across arthropods and beyond.

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.

Simplet-dependent regulation of β-catenin signaling influences skeletal patterning downstream of Cx43

The correct positioning of joints in the vertebrate skeleton is not well understood. Mutations in connexin43 (cx43) cause the short segment phenotype of the zebrafish short fin (sof^b123 ) mutant. We have shown that Cx43 suppresses evx1 expression, a transcription factor required for joint formation. Here, we provide novel insights into how Cx43 influences evx1 transcription. First, we find that Simplet (Smp) knockdown recapitulates the sof^b123 phenotypes of reduced regenerate length and reduced segment length, and we find evidence for synergy between cx43 and smp Moreover, knockdown of Smp increases the evx1 expression, similar to cx43 knockdown. Previous studies have shown that Smp is required for the nuclear localization of β-catenin. Indeed, β-catenin activity is required for segment length, and is reduced in both sof^b123 mutants and following Smp knockdown in regenerating fins. We further show that blocking canonical Wnt signaling results in a synergistic reduction in segment length in sof^{b123 /+} heterozygotes. Together, our findings suggest that both Smp and β-catenin function in a common molecular pathway with cx43 to influence both evx1 expression and joint location.

Tissue Regeneration Enhancer Elements: A Way to Unlock Endogenous Healing Power

Regenerative capacity is widespread throughout almost all animal phyla. However, the distribution pattern remains incompletely understood. Various examples show that very closely related species display different regenerative capacities. Why and how have diverse regenerative capacities evolved across species? One prevailing thought in the field of regeneration is that most regeneration-associated factors are evolutionarily conserved, suggesting the existence of an innate tissue regeneration ability in all species. However, its regulation is differentially controlled in distinct species, resulting in heterogeneous regenerative capabilities. In this review, we discuss regeneration-associated enhancers, the key cis-regulatory elements controlling gene expression, their underlying molecular mechanisms, and their influence on regenerative capacity. Understanding the regulatory mechanisms of regeneration enhancers can provide fundamental insights into tissue regeneration and further help us develop therapeutic strategies to unlock latent healing powers in humans. Developmental Dynamics 248:34-42, 2019. © 2018 Wiley Periodicals, Inc.

Basal constriction during midbrain-hindbrain boundary morphogenesis is mediated by Wnt5b and focal adhesion kinase

Basal constriction occurs at the zebrafish midbrain–hindbrain boundary constriction (MHBC) and is likely a widespread morphogenetic mechanism. 3D reconstruction demonstrates that MHBC cells are wedge-shaped, and initially constrict basally, with subsequent apical expansion. wnt5b is expressed in the MHB and is required for basal constriction. Consistent with a requirement for this pathway, expression of dominant negative Gsk3β overcomes wnt5b knockdown. Immunostaining identifies focal adhesion kinase (Fak) as active in the MHB region, and knockdown demonstrates Fak is a regulator of basal constriction. Tissue specific knockdown further indicates that Fak functions cell autonomously within the MHBC. Fak acts downstream of wnt5b, suggesting that Wnt5b signals locally as an early step in basal constriction and acts together with more widespread Fak activation. This study delineates signaling pathways that regulate basal constriction during brain morphogenesis.

Summary: Focal adhesion kinase acts downstream of Wnt5b to mediate basal constriction of neuroepithelial cells during the formation of the midbrain–hindbrain boundary.

Damage-induced reactive oxygen species enable zebrafish tail regeneration by repositioning of Hedgehog expressing cells

Many aquatic vertebrates have a remarkable ability to regenerate limbs and tails after amputation. Previous studies indicate that reactive oxygen species (ROS) signalling initiates regeneration, but the mechanism by which this takes place is poorly understood. Developmental signalling pathways have been shown to have proregenerative roles in many systems. However, whether these are playing roles that are specific to regeneration, or are simply recapitulating their developmental functions is unclear. Here, we analyse zebrafish larval tail regeneration and find evidence that ROS released upon wounding cause repositioning of notochord cells to the damage site. These cells secrete Hedgehog ligands that are required for regeneration. Hedgehog signalling is not required for normal tail development suggesting that it has a regeneration-specific role. Our results provide a model for how ROS initiate tail regeneration, and indicate that developmental signalling pathways can play regenerative functions that are not directly related to their developmental roles.

Reactive oxygen species (ROS) are required to initiate regeneration but the mechanisms regulating its production are unclear. Here, the authors show in zebrafish larval tail regeneration that ROS is released by mobilised notochord cells enables their repositioning in the damage site, assisted by secreted Hh.

Hedgehog and Wnt Signaling Pathways Regulate Tail Regeneration

Urodele amphibians have a tremendous capacity for the regeneration of appendages, including limb and tail, following injury. While studies have focused on the cellular and morphological changes during appendicular regeneration, the signaling mechanisms that govern these cytoarchitectural changes during the regenerative response are unclear. In this study, we describe the essential role of hedgehog (Hh) and Wnt signaling pathways following tail amputation in the newt. Quantitative PCR studies revealed that members of both the Hh and Wnt signaling pathways, including the following: shh, ihh, ptc-1, wnt-3a, β-catenin, axin2, frizzled (frzd)-1, and frzd-2 transcripts, were induced following injury. Continuous pharmacological-mediated inhibition of Hh signaling resulted in spike-like regenerates with no evidence of tissue patterning, whereas activation of Hh signaling enhanced the regenerative process. Pharmacological-mediated temporal inhibition experiments demonstrated that the Hh-mediated patterning of the regenerating tail occurs early during regeneration and Hh signals are continuously required for proliferation of the blastemal progenitors. BrdU incorporation and PCNA immunohistochemical studies demonstrated that Hh signaling regulates the cellular proliferation of the blastemal cells following amputation. Similarly, Wnt inhibition resulted in perturbed regeneration, whereas its activation promoted tail regeneration. Using an inhibitor-activator strategy, we demonstrated that the Wnt pathway is likely to be upstream of the Hh pathway and together these signaling pathways function in a coordinated manner to facilitate tail regeneration. Mechanistically, the Wnt signaling pathway activated the Hh signaling pathway that included ihh and ptc-1 during the tail regenerative process. Collectively, our results demonstrate the absolute requirement of signaling pathways that are essential in the regulation of tail regeneration.

Regulation of Osteoblast Metabolism by Wnt Signaling

Wnt/β-catenin signaling plays a critical role in the achievement of peak bone mass, affecting the commitment of mesenchymal progenitors to the osteoblast lineage and the anabolic capacity of osteoblasts depositing bone matrix. Recent studies suggest that this evolutionarily-conserved, developmental pathway exerts its anabolic effects in part by coordinating osteoblast activity with intermediary metabolism. These findings are compatible with the cloning of the gene encoding the low-density lipoprotein related receptor-5 (LRP5) Wnt co-receptor from a diabetes-susceptibility locus and the now well-established linkage between Wnt signaling and metabolism. In this article, we provide an overview of the role of Wnt signaling in whole-body metabolism and review the literature regarding the impact of Wnt signaling on the osteoblast's utilization of three different energy sources: fatty acids, glucose, and glutamine. Special attention is devoted to the net effect of nutrient utilization and the mode of regulation by Wnt signaling. Mechanistic studies indicate that the utilization of each substrate is governed by a unique mechanism of control with β-catenin-dependent signaling regulating fatty acid β-oxidation, while glucose and glutamine utilization are β-catenin-independent and downstream of mammalian target of rapamycin complex 2 (mTORC2) and mammalian target of rapamycin complex 1 (mTORC1) activation, respectively. The emergence of these data has provided a new context for the mechanisms by which Wnt signaling influences bone development.

Cxcl12a induces snail1b expression to initiate collective migration and sequential Fgf-dependent neuromast formation in the zebrafish posterior lateral line primordium

The zebrafish posterior lateral line primordium migrates along a path defined by the chemokine Cxcl12a, periodically depositing neuromasts, to pioneer formation of the zebrafish posterior lateral line system. snail1b, known for its role in promoting cell migration, is expressed in leading cells of the primordium in response to Cxcl12a, whereas its expression in trailing cells is inhibited by Fgf signaling. snail1b knockdown delays initiation of primordium migration. This delay is associated with aberrant expansion of epithelial cell adhesion molecule (epcam) and reduction of cadherin 2 expression in the leading part of the primordium. Co-injection of snail1b morpholino with snail1b mRNA prevents the initial delay in migration and restores normal expression of epcam and cadherin 2 The delay in initiating primordium migration in snail1b morphants is accompanied by a delay in sequential formation of trailing Fgf signaling centers and associated protoneuromasts. This delay is not specifically associated with knockdown of snail1b but also with other manipulations that delay migration of the primordium. These observations reveal an unexpected link between the initiation of collective migration and sequential formation of protoneuromasts in the primordium.

Wnt/β-catenin regulates an ancient signaling network during zebrafish scale development

Understanding how patterning influences cell behaviors to generate three dimensional morphologies is a central goal of developmental biology. Additionally, comparing these regulatory mechanisms among morphologically diverse tissues allows for rigorous testing of evolutionary hypotheses. Zebrafish skin is endowed with a coat of precisely patterned bony scales. We use in-toto live imaging during scale development and manipulations of cell signaling activity to elucidate core features of scale patterning and morphogenesis. These analyses show that scale development requires the concerted activity of Wnt/β-catenin, Ectodysplasin (Eda) and Fibroblast growth factor (Fgf) signaling. This regulatory module coordinates Hedgehog (HH) dependent collective cell migration during epidermal invagination, a cell behavior not previously implicated in skin appendage morphogenesis. Our analyses demonstrate the utility of zebrafish scale development as a tractable system in which to elucidate mechanisms of developmental patterning and morphogenesis, and suggest a single, ancient origin of skin appendage patterning mechanisms in vertebrates.

Wnt signalling: conquering complexity

The history of the Wnt pathway is an adventure that takes us from mice and flies to frogs, zebrafish and beyond, sketching the outlines of a molecular signalling cascade along the way. Here, we specifically highlight the instrumental role that developmental biology has played throughout. We take the reader on a journey, starting with developmental genetics studies that identified some of the main molecular players, through developmental model organisms that helped unravel their biochemical function and cell biological activities. Culminating in complex analyses of stem cell fate and dynamic tissue growth, these efforts beautifully illustrate how different disciplines provided missing pieces of a puzzle. Together, they have shaped our mechanistic understanding of the Wnt pathway as a conserved signalling process in development and disease. Today, researchers are still uncovering additional roles for Wnts and other members of this multifaceted signal transduction pathway, opening up promising new avenues for clinical applications.

Analysis of a conditional gene trap reveals that tbx5a is required for heart regeneration in zebrafish

The ability to conditionally inactivate genes is instrumental for fine genetic analysis of all biological processes, but is especially important for studies of biological events, such as regeneration, which occur late in ontogenesis or in adult life. We have constructed and tested a fully conditional gene trap vector, and used it to inactivate tbx5a in the cardiomyocytes of larval and adult zebrafish. We observe that loss of tbx5a function significantly impairs the ability of zebrafish hearts to regenerate after ventricular resection, indicating that Tbx5a plays an essential role in the transcriptional program of heart regeneration.

The role of neuro-epithelial-like and radial-glial stem and progenitor cells in development, plasticity, and repair

Neural stem and progenitor cells (NSPCs) are the primary source of new neurons in the brain and serve critical roles in tissue homeostasis and plasticity throughout life. Within the vertebrate brain, NSPCs are located within distinct neurogenic niches differing in their location, cellular composition, and proliferative behaviour. Heterogeneity in the NSPC population is hypothesized to reflect varying capacities for neurogenesis, plasticity and repair between different neurogenic zones. Since the discovery of adult neurogenesis, studies have predominantly focused on the behaviour and biological significance of adult NSPCs (aNSPCs) in rodents. However, compared to rodents, who show lifelong neurogenesis in only two restricted neurogenic niches, zebrafish exhibit constitutive neurogenesis across multiple stem cell niches that provide new neurons to every major brain division. Accordingly, zebrafish are a powerful model to probe the unique cellular and molecular profiles of NSPCs and investigate how these profiles govern tissue homeostasis and regenerative plasticity within distinct stem cell populations over time. Amongst the NSPC populations residing in the zebrafish central nervous system (CNS), proliferating radial-glia, quiescent radial-glia and neuro-epithelial-like cells comprise the majority. Here, we provide insight into the extent to which these distinct NSPC populations function and mature during development, respond to experience, and contribute to successful CNS regeneration in teleost fish. Together, our review brings to light the dynamic biological roles of these individual NSPC populations and showcases their diverse regenerative modes to achieve vertebrate brain repair later in life.

RegenDbase: a comparative database of noncoding RNA regulation of tissue regeneration circuits across multiple taxa

Regeneration is an endogenous process of tissue repair that culminates in complete restoration of tissue and organ function. While regenerative capacity in mammals is limited to select tissues, lower vertebrates like zebrafish and salamanders are endowed with the capacity to regenerate entire limbs and most adult tissues, including heart muscle. Numerous profiling studies have been conducted using these research models in an effort to identify the genetic circuits that accompany tissue regeneration. Most of these studies, however, are confined to an individual injury model and/or research organism and focused primarily on protein encoding transcripts. Here we describe RegenDbase, a new database with the functionality to compare and contrast gene regulatory pathways within and across tissues and research models. RegenDbase combines pipelines that integrate analysis of noncoding RNAs in combination with protein encoding transcripts. We created RegenDbase with a newly generated comprehensive dataset for adult zebrafish heart regeneration combined with existing microarray and RNA-sequencing studies on multiple injured tissues. In this current release, we detail microRNA-mRNA regulatory circuits and the biological processes these interactions control during the early stages of heart regeneration. Moreover, we identify known and putative novel lncRNAs and identify their potential target genes based on proximity searches. We postulate that these candidate factors underscore robust regenerative capacity in lower vertebrates. RegenDbase provides a systems-level analysis of tissue regeneration genetic circuits across injury and animal models and addresses the growing need to understand how noncoding RNAs influence these changes in gene expression.

Wound-induced cell proliferation during animal regeneration

Many animal species are capable of replacing missing tissues that are lost upon injury or amputation through the process of regeneration. Although the extent of regeneration is variable across animals, that is, some animals can regenerate any missing cell type whereas some can only regenerate certain organs or tissues, regulated cell proliferation underlies the formation of new tissues in most systems. Notably, many species display an increase in proliferation within hours or days upon wounding. While different cell types proliferate in response to wounding in various animal taxa, comparative molecular data are beginning to point to shared wound-induced mechanisms that regulate cell division during regeneration. Here, we synthesize current insights about early molecular pathways of regeneration from diverse model and emerging systems by considering these species in their evolutionary contexts. Despite the great diversity of mechanisms underlying injury-induced cell proliferation across animals, and sometimes even in the same species, similar pathways for proliferation have been implicated in distantly related species (e.g., small diffusible molecules, signaling from apoptotic cells, growth factor signaling, mTOR and Hippo signaling, and Wnt and Bmp pathways). Studies that explicitly interrogate molecular and cellular regenerative mechanisms in understudied animal phyla will reveal the extent to which early pathways in the process of regeneration are conserved or independently evolved. This article is categorized under: Comparative Development and Evolution > Body Plan Evolution Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration Comparative Development and Evolution > Model Systems.

De novo draft assembly of the Botrylloides leachii genome provides further insight into tunicate evolution

Tunicates are marine invertebrates that compose the closest phylogenetic group to the vertebrates. These chordates present a particularly diverse range of regenerative abilities and life-history strategies. Consequently, tunicates provide an extraordinary perspective into the emergence and diversity of these traits. Here we describe the genome sequencing, annotation and analysis of the Stolidobranchian Botrylloides leachii. We have produced a high-quality 159 Mb assembly, 82% of the predicted 194 Mb genome. Analysing genome size, gene number, repetitive elements, orthologs clustering and gene ontology terms show that B. leachii has a genomic architecture similar to that of most solitary tunicates, while other recently sequenced colonial ascidians have undergone genome expansion. In addition, ortholog clustering has identified groups of candidate genes for the study of colonialism and whole-body regeneration. By analysing the structure and composition of conserved gene linkages, we observed examples of cluster breaks and gene dispersions, suggesting that several lineage-specific genome rearrangements occurred during tunicate evolution. We also found lineage-specific gene gain and loss within conserved cell-signalling pathways. Such examples of genetic changes within conserved cell-signalling pathways commonly associated with regeneration and development that may underlie some of the diverse regenerative abilities observed in tunicates. Overall, these results provide a novel resource for the study of tunicates and of colonial ascidians.

Drosophila as a Model System to Study Cell Signaling in Organ Regeneration

Regeneration is a fascinating phenomenon that allows organisms to replace or repair damaged organs or tissues. This ability occurs to varying extents among metazoans. The rebuilding of the damaged structure depends on regenerative proliferation that must be accompanied by proper cell fate respecification and patterning. These cellular processes are regulated by the action of different signaling pathways that are activated in response to the damage. The imaginal discs of Drosophila melanogaster have the ability to regenerate and have been extensively used as a model system to study regeneration. Drosophila provides an opportunity to use powerful genetic tools to address fundamental problems about the genetic mechanisms involved in organ regeneration. Different studies in Drosophila have helped to elucidate the genes and signaling pathways that initiate regeneration, promote regenerative growth, and induce cell fate respecification. Here we review the signaling networks involved in regulating the variety of cellular responses that are required for discs regeneration.

Comparative regenerative mechanisms across different mammalian tissues

Stimulating regeneration of complex tissues and organs after injury to effect complete structural and functional repair, is an attractive therapeutic option that would revolutionize clinical medicine. Compared to many metazoan phyla that show extraordinary regenerative capacity, which in some instances persists throughout life, regeneration in mammalians, particularly humans, is limited or absent. Here we consider recent insights in the elucidation of molecular mechanisms of regeneration that have come from studies of tissue homeostasis and injury repair in mammalian tissues that span the spectrum from little or no self-renewal, to those showing active cell turnover throughout life. These studies highlight the diversity of factors that constrain regeneration, including immune responses, extracellular matrix composition, age, injury type, physiological adaptation, and angiogenic and neurogenic capacity. Despite these constraints, much progress has been made in elucidating key molecular mechanisms that may provide therapeutic targets for the development of future regenerative therapies, as well as previously unidentified developmental paradigms and windows-of-opportunity for improved regenerative repair.

Widening control of fin inter-rays in zebrafish and inferences about actinopterygian fins

The amputation of a teleost fin rapidly triggers an intricate maze of hierarchically regulated signalling processes which ultimately reconstruct the diverse tissues of the appendage. Whereas the generation of the fin pattern along the proximodistal axis brings with it several well-known developmental regulators, the mechanisms by which the fin widens along its dorsoventral axis remain poorly understood. Utilizing the zebrafish as an experimental model of fin regeneration and studying more than 1000 actinopterygian species, we hypothesized a connection between specific inter-ray regulatory mechanisms and the morphological variability of inter-ray membranes found in nature. To tackle these issues, both cellular and molecular approaches have been adopted and our results suggest the existence of two distinguishable inter-ray areas in the zebrafish caudal fin, a marginal and a central region. The present work associates the activity of the cell membrane potassium channel kcnk5b, the fibroblast growth factor receptor 1 and the sonic hedgehog pathway to the control of several cell functions involved in inter-ray wound healing or dorsoventral regeneration of the zebrafish caudal fin. This ray-dependent regulation controls cell migration, cell-type patterning and gene expression. The possibility that modifications of these mechanisms are responsible for phenotypic variations found in euteleostean species, is discussed.

Wnt signaling regulates proliferation and differentiation of radial glia in regenerative processes after stab injury in the optic tectum of adult zebrafish

Zebrafish have superior abilities to generate new neurons in the adult brain and to regenerate brain tissue after brain injury compared with mammals. There exist two types of neural stem cells (NSCs): neuroepithelial-like stem cells (NE) and radial glia (RG) in the optic tectum. We established an optic tectum stab injury model to analyze the function of NSCs in the regenerative condition and confirmed that the injury induced the proliferation of RG, but not NE and that the proliferated RG differentiated into new neurons after the injury. We then analyzed the involvement of Wnt signaling after the injury, using a Wnt reporter line in which canonical Wnt signaling activation induced GFP expression and confirmed that GFP expression was induced specifically in RG after the injury. We also analyzed the expression level of genes related to Wnt signaling, and confirmed that endogenous Wnt antagonist dkk1b expression was significantly decreased after the injury. We observed that Wnt signal inhibitor IWR1 treatment suppressed the proliferation and differentiation of RG after the injury, suggesting that up-regulation of Wnt signaling in RG after the stab injury was required for optic tectum regeneration. We also confirmed that Wnt activation by treatment with GSK3β inhibitor BIO in uninjured zebrafish induced proliferation of RG in the optic tectum. This optic tectum stab injury model is useful for the study of the molecular mechanisms of brain regeneration and analysis of the RG functions in physiological and regenerative conditions.

WNT Signaling in Cardiac and Vascular Disease

WNT signaling is an elaborate and complex collection of signal transduction pathways mediated by multiple signaling molecules. WNT signaling is critically important for developmental processes, including cell proliferation, differentiation and tissue patterning. Little WNT signaling activity is present in the cardiovascular system of healthy adults, but reactivation of the pathway is observed in many pathologies of heart and blood vessels. The high prevalence of these pathologies and their significant contribution to human disease burden has raised interest in WNT signaling as a potential target for therapeutic intervention. In this review, we first will focus on the constituents of the pathway and their regulation and the different signaling routes. Subsequently, the role of WNT signaling in cardiovascular development is addressed, followed by a detailed discussion of its involvement in vascular and cardiac disease. After highlighting the crosstalk between WNT, transforming growth factor-β and angiotensin II signaling, and the emerging role of WNT signaling in the regulation of stem cells, we provide an overview of drugs targeting the pathway at different levels. From the combined studies we conclude that, despite the sometimes conflicting experimental data, a general picture is emerging that excessive stimulation of WNT signaling adversely affects cardiovascular pathology. The rapidly increasing collection of drugs interfering at different levels of WNT signaling will allow the evaluation of therapeutic interventions in the pathway in relevant animal models of cardiovascular diseases and eventually in patients in the near future, translating the outcomes of the many preclinical studies into a clinically relevant context.

β-Catenin Directs Long-Chain Fatty Acid Catabolism in the Osteoblasts of Male Mice

Wnt-initiated signaling through a frizzled receptor and the low-density lipoprotein-related receptor-5 coreceptor instructs key anabolic events during skeletal development, homeostasis, and repair. Recent studies indicate that Wnt signaling also regulates the intermediary metabolism of osteoblastic cells, inducing glucose consumption in osteoprogenitors and fatty acid utilization in mature osteoblasts. In this study, we examined the role of the canonical Wnt-signaling target, β-catenin, in the control of osteoblast metabolism. In vitro, Wnt ligands and agonists that stimulated β-catenin activation in osteoblasts enhanced fatty acid catabolism, whereas genetic ablation of β-catenin dramatically reduced oleate oxidation concomitant with reduced osteoblast maturation and increased glycolytic metabolism. Temporal ablation of β-catenin expression in osteoblasts in vivo produced the expected low-bone-mass phenotype and also led to an increase in white adipose tissue mass, dyslipidemia, and impaired insulin sensitivity. Because the expression levels of enzymatic mediators of fatty acid β-oxidation are reduced in the skeleton of β-catenin mutants, these results further confirm the role of the osteoblast in lipid metabolism and indicate that the influence of Wnt signaling on fatty acid utilization proceeds via its canonical signaling pathway.

Generic wound signals initiate regeneration in missing-tissue contexts

Despite the identification of numerous regulators of regeneration in different animal models, a fundamental question remains: why do some wounds trigger the full regeneration of lost body parts, whereas others resolve by mere healing? By selectively inhibiting regeneration initiation, but not the formation of a wound epidermis, here we create headless planarians and finless zebrafish. Strikingly, in both missing-tissue contexts, injuries that normally do not trigger regeneration activate complete restoration of heads and fin rays. Our results demonstrate that generic wound signals have regeneration-inducing power. However, they are interpreted as regeneration triggers only in a permissive tissue context: when body parts are missing, or when tissue-resident polarity signals, such as Wnt activity in planarians, are modified. Hence, the ability to decode generic wound-induced signals as regeneration-initiating cues may be the crucial difference that distinguishes animals that regenerate from those that cannot.

Some wounds trigger regeneration, while others simply heal but how this is regulated is unclear. Here, by manipulating ERK and Wnt signalling pathways, the authors create headless planarians and finless zebrafish and show that wounds that normally only trigger wound healing can activate regeneration of heads and bones.

The role of gastrulation brain homeobox 2 (gbx2) in the development of the ventral telencephalon in zebrafish embryos

During vertebrate brain development, the gastrulation brain homeobox 2 gene (gbx2) is expressed in the forebrain, but its precise roles are still unknown. In this study, we addressed this issue in zebrafish (Danio rerio) first by carefully examining gbx2 expression in the developing forebrain. We showed that gbx2 was expressed in the telencephalon during late somitogenesis, from 18h post-fertilization (hpf) to 24 hpf, and in the thalamic primordium after 26 hpf. In contrast, another gbx gene, gbx1, was expressed in the anterior-most ventral telencephalon after 36 hpf. Thus, the expression patterns of these two gbx genes did not overlap, arguing against their redundant function in the forebrain. Two-color fluorescence in situ hybridization (FISH) showed close relationships between the telencephalic expression of gbx2 and other forebrain-forming genes, suggesting that their interactions contribute to the regionalization of the telencephalon. FISH further revealed that gbx2 is expressed in the ventricular region of the telencephalon. By using transgenic fish in which gbx2 can be induced by heat shock, we found that gbx2 induction at 16 hpf repressed the expression of emx3, dlx2a, and six3b in the ventral telencephalon. Among secreted factor genes, bmp2b and wnt1 were repressed in the vicinity of the gbx2 domain in the telencephalon. The expression of forebrain-forming genes was examined in mutant embryos lacking gbx2, showing emx3 and dlx2a to be upregulated in the subpallium at 24 hpf. Taken together, these findings indicate that gbx2 contributes to the development of the subpallium through its repressive activities against other telencephalon-forming genes. We further showed that inhibiting FGF signaling and activating Wnt signaling repressed gbx2 and affected the regionalization of the telencephalon, supporting a functional link between gbx2, intracellular signaling, and telencephalon development.

Skin and Its Regenerative Powers: An Alliance between Stem Cells and Their Niche

Tissues have a natural capacity to replace dying cells and to heal wounds. This ability resides in resident stem cells, which self-renew, preserve, and repair their tissue during homeostasis and following injury. The skin epidermis and its appendages are subjected to daily assaults from the external environment. A high demand is placed on renewal and regeneration of the skin's barrier in order to protect the body from infection and dehydration and to heal wounds. This review focuses on the epithelial stem cells of skin, where they come from, where they reside, and how they function in normal homeostasis and wound repair.

A gene expression study of ornamental fin shape in Neolamprologus brichardi, an African cichlid species

The diversity of fin morphology within and across fish taxa offers great, but still largely unexplored, opportunities to investigate the proximate mechanisms underlying fin shape variation. Relying on available genetic knowledge brought forth mainly by the comprehensive study of the zebrafish caudal fin, we explored candidate molecular mechanisms for the maintenance and formation of the conspicuously elongated filaments adorning the unpaired fins of the East African "princess cichlid" Neolamprologus brichardi. Via qPCR assays, we detected expression differences of candidate genes between elongated and short regions of intact and regenerating fins. The identified genes include skeletogenic and growth factors (igf2b, fgf3, bmp2 and bmp4), components of the WNT pathway (lef1, wnt5b and wnt10) and a regulatory network determining fin ray segment size and junction (cx43, esco2 and sema3d), as well as other genes with different roles (mmp9, msxb and pea3). Interestingly, some of these genes showed fin specific expression differences which are often neglected in studies of model fish that focus on the caudal fin. Moreover, while the observed expression patterns were generally consistent with zebrafish results, we also detected deviating expression correlations and gene functions.

Regeneration Genetics

Understanding how and why animals regenerate complex tissues has the potential to transform regenerative medicine. Here we present an overview of genetic approaches that have recently been applied to dissect mechanisms of regeneration. We describe new advances that relate to central objectives of regeneration biologists researching different tissues and species, focusing mainly on vertebrates. These objectives include defining the cellular sources and key cell behaviors in regenerating tissue, elucidating molecular triggers and brakes for regeneration, and defining the earliest events that control the presence of these molecular factors.

Cellular and molecular mechanisms underlying planar cell polarity pathway contributions to cancer malignancy

While the mutational activation of oncogenes drives tumor initiation and growth by promoting cellular transformation and proliferation, increasing evidence suggests that the subsequent re-engagement of largely dormant developmental pathways contributes to cellular phenotypes associated with the malignancy of solid tumors. Genetic studies from a variety of model organisms have defined many of the components that maintain epithelial planar cell polarity (PCP), or cellular polarity in the axis orthogonal to the apical-basal axis. These same components comprise an arm of non-canonical Wnt signaling that mediates cell motility events such as convergent extension movements essential to proper development. In this review, we summarize the increasing evidence that the Wnt/PCP signaling pathway plays active roles in promoting the proliferative and migratory properties of tumor cells, emphasizing the importance of subcellular localization of PCP components and protein-protein interactions in regulating cellullar properties associated with malignancy. Specifically, we discuss the increased expression of Wnt/PCP pathway components in cancer and the functional consequences of aberrant pathway activation, focusing on Wnt ligands, Frizzled (Fzd) receptors, the tetraspanin-like proteins Vangl1 and Vangl2, and the Prickle1 (Pk1) scaffold protein. In addition, we discuss negative regulation of the Wnt/PCP pathway, with particular emphasis on the Nrdp1 E3 ubiquitin ligase. We hypothesize that engagement of the Wnt/PCP pathway after tumor initiation drives malignancy by promoting cellular proliferation and invasiveness, and that the ability of Wnt/PCP signaling to supplant oncogene addiction may contribute to tumor resistance to oncogenic pathway-directed therapeutic agents.

Antagonism between Hedgehog and Wnt signaling pathways regulates tumorigenicity

The crosstalk of multiple cellular signaling pathways is crucial in animal development and tissue homeostasis, and its dysregulation may result in tumor formation and metastasis. The Hedgehog (Hh) and Wnt signaling pathways are both considered to be essential regulators of cell proliferation, differentiation and oncogenesis. Recent studies have indicated that the Hh and Wnt signaling pathways are closely associated and involved in regulating embryogenesis and cellular differentiation. Hh signaling acts upstream of the Wnt signaling pathway, and negative regulates Wnt activity via secreted frizzled-related protein 1 (SFRP1), and the Wnt/β-catenin pathway downregulates Hh activity through glioma-associated oncogene homolog 3 transcriptional regulation. This evidence suggests that the imbalance of Hh and Wnt regulation serves a crucial role in cancer-associated processes. The activation of SFRP1, which inhibits Wnt, has been demonstrated to be an important cross-point between the two signaling pathways. The present study reviews the complex interaction between the Hh and Wnt signaling pathways in embryogenesis and tumorigenicity, and the role of SFRP1 as an important mediator associated with the dysregulation of the Hh and Wnt signaling pathways.

Retinoic acid is required and Fgf, Wnt, and Bmp signaling inhibit posterior lateral line placode induction in zebrafish

The lateral line system is a mechanosensory systems present in aquatic animals. The anterior and posterior lateral lines develop from anterior and posterior lateral line placodes (aLLp and pLLp), respectively. Although signaling molecules required for the induction of other cranial placodes have been well studied, the molecular mechanisms underlying formation of the lateral line placodes are unknown. In this study we tested the requirement of multiple signaling pathways, such as Wnt, Bmp Fgf, and Retinoic Acid for aLLp and pLLp induction. We determined that aLLp specification requires Fgf signaling, whilst pLLp specification requires retinoic acid which inhibits Fgf signaling. pLLp induction is also independent of Wnt and Bmp activities, even though these pathways limit the boundaries of the pLLp. This is the first report that the aLLp and pLLp depend on different inductive mechanisms and that pLLp induction requires the inhibition of Fgf, Wnt and Bmp signaling.

BMP signaling regulates the skeletal and connective tissue differentiation during caudal fin regeneration in sailfin molly (Poecilia latipinna)

Caudal fin regeneration in sailfin molly, Poecilia latipinna (Lesueur 1821) involves an initial wound healing stage, followed by blastema that is formed of fast proliferating cells. In order to replicate the lost fin, correct differentiation of the blastemal cells into various tissues is the prime essence. Among the molecular signals governing proper differentiation of blastemal cells, members of the bone morphogenetic protein (BMP) family are crucial. Herein, we investigated the specific effects of inhibition of BMP signaling using LDN193189 on skeletal and connective tissue formation in the regenerating tail fin of P. latipinna during early differentiation phase. It was observed that BMP inhibition leads to reduction in the length of regeneration, which can be correlated with compromised proliferation of blastemal cells. Decreased expression of cell proliferation marker like pcna together with reduced BrdU positive cells consolidate the above observation. Further, histological analysis revealed stunted progression of skeletal tissues and this correlated with the reduced expression of sox9, runx2 and dlx5, Osc and Osn genes in response to BMP inhibition. Also, defective bone patterning was observed due to BMP inhibition, which was associated with diminished levels of shh, ptc-1, gli2 and other BMP ligands. Moreover, histochemical analysis revealed that collagen, one of the most prominent components of connective tissue, was formed below par in treated fin tissues which was subsequently confirmed by biochemical and transcript level analyses. Overall our results highlight the importance of the BMP pathway in proper differentiation of skeletal and connective tissues during the differentiation stage of regenerating caudal fin.

Mechanisms of urodele limb regeneration

This review explores the historical and current state of our knowledge about urodele limb regeneration. Topics discussed are (1) blastema formation by the proteolytic histolysis of limb tissues to release resident stem cells and mononucleate cells that undergo dedifferentiation, cell cycle entry and accumulation under the apical epidermal cap. (2) The origin, phenotypic memory, and positional memory of blastema cells. (3) The role played by macrophages in the early events of regeneration. (4) The role of neural and AEC factors and interaction between blastema cells in mitosis and distalization. (5) Models of pattern formation based on the results of axial reversal experiments, experiments on the regeneration of half and double half limbs, and experiments using retinoic acid to alter positional identity of blastema cells. (6) Possible mechanisms of distalization during normal and intercalary regeneration. (7) Is pattern formation is a self-organizing property of the blastema or dictated by chemical signals from adjacent tissues? (8) What is the future for regenerating a human limb?

Animal regeneration: ancestral character or evolutionary novelty?

An old question about regeneration is whether it is an ancestral character which is a general property of living matter, or whether it represents a set of specific adaptations to the different circumstances faced by different types of animal. In this review, some recent results on regeneration are assessed to see if they can throw any new light on this question. Evidence in favour of an ancestral character comes from the role of Wnt and bone morphogenetic protein signalling in controlling the pattern of whole-body regeneration in acoels, which are a basal group of bilaterian animals. On the other hand, there is some evidence for adaptive acquisition or maintenance of the regeneration of appendages based on the occurrence of severe non-lethal predation, the existence of some novel genes in regenerating organisms, and differences at the molecular level between apparently similar forms of regeneration. It is tentatively concluded that whole-body regeneration is an ancestral character although has been lost from most animal lineages. Appendage regeneration is more likely to represent a derived character resulting from many specific adaptations.

Wnt signaling controls pro-regenerative Collagen XII in functional spinal cord regeneration in zebrafish

The inhibitory extracellular matrix in a spinal lesion site is a major impediment to axonal regeneration in mammals. In contrast, the extracellular matrix in zebrafish allows substantial axon re-growth, leading to recovery of movement. However, little is known about regulation and composition of the growth-promoting extracellular matrix. Here we demonstrate that activity of the Wnt/β-catenin pathway in fibroblast-like cells in the lesion site is pivotal for axon re-growth and functional recovery. Wnt/β-catenin signaling induces expression of col12a1a/b and deposition of Collagen XII, which is necessary for axons to actively navigate the non-neural lesion site environment. Overexpression of col12a1a rescues the effects of Wnt/β-catenin pathway inhibition and is sufficient to accelerate regeneration. We demonstrate that in a vertebrate of high regenerative capacity, Wnt/β-catenin signaling controls the composition of the lesion site extracellular matrix and we identify Collagen XII as a promoter of axonal regeneration. These findings imply that the Wnt/β-catenin pathway and Collagen XII may be targets for extracellular matrix manipulations in non-regenerating species.

Following spinal injury in zebrafish, non-neural cells establish an extracellular matrix to promote axon re-growth but how this is regulated is unclear. Here, the authors show that Wnt/β-catenin signaling in fibroblast-like cells at a lesion activates axon re-growth via deposition of Collagen XII.

Harmless effects of argon plasma on caudal fin regeneration and embryogenesis of zebrafish: novel biological approaches for safe medical applications of bioplasma

The argon plasma jet (Ar-PJ) is widely used in medical fields such as dermatology and dentistry, and it is considered a promising tool for cancer therapy. However, the in vivo effects of Ar-PJ for medical uses have not yet been investigated, and there are no biological tools to determine the appropriate clinical dosages of Ar-PJ. In this study, we used the caudal fin and embryo of zebrafish as novel in vivo tools to evaluate the biosafety of Ar-PJ. Typically, Ar-PJ is known to induce cell death in two-dimensional (2D) cell culture systems. By contrast, no detrimental effects of Ar-PJ were shown in our 3D zebrafish systems composed of 2D cells. The Ar-PJ-treated caudal fins grew by an average length of 0.7 mm, similar to the length of the normally regenerating fins. Remarkably, Ar-PJ did not affect the expression patterns of Wnt8a and β-Catenin, which play important roles in fin regeneration. In the embryo system, 85% of the Ar-PJ-treated embryos hatched, and the lateral length of these embryos was ~3.3 mm, which are equivalent to the lengths of normal embryos. In particular, vasculogenesis, which is the main cellular process during tissue regeneration and embryogenesis, occurred normally under the Ar-PJ dose used in this study. Therefore, our biosafety evaluation tools that use living model systems can be used to provide an experimental guideline to determine the clinically safe dosage of Ar-PJ.

Convergence of signaling pathways underlying habenular formation and axonal outgrowth in zebrafish

The habenular nuclei are a conserved integrating center in the vertebrate epithalamus, where they modulate diverse behaviors. Despite their importance, our understanding of habenular development is incomplete. Time-lapse imaging and fate mapping demonstrate that the dorsal habenulae (dHb) of zebrafish are derived from dbx1b-expressing (dbx1b⁺ ) progenitors, which transition into cxcr4b-expressing neuronal precursors. The precursors give rise to differentiated neurons, the axons of which innervate the midbrain interpeduncular nucleus (IPN). Formation of the dbx1b⁺ progenitor population relies on the activity of the Shh, Wnt and Fgf signaling pathways. Wnt and Fgf function additively to generate dHb progenitors. Surprisingly, Wnt signaling also negatively regulates fgf8a, confining expression to a discrete dorsal diencephalic domain. Moreover, the Wnt and Fgf pathways have opposing roles in transcriptional regulation of components of the Cxcr4-chemokine signaling pathway. The chemokine pathway, in turn, directs the posterior outgrowth of dHb efferents toward the IPN and, when disrupted, results in ectopic, anteriorly directed axonal projections. The results define a signaling network underlying the generation of dHb neurons and connectivity with their midbrain target.

LRH-1 senses signaling from phosphatidylcholine to regulate the expansion growth of digestive organs via synergy with Wnt/β-catenin signaling in zebrafish

Liver receptor homolog-1 (LRH-1) is an orphan nuclear receptor that is critical for the growth and proliferation of cancer cells and other biological processes, including lipid transportation and metabolism, sexual determination and steroidogenesis. However, because homozygous lrh-1^-/- mice die in utero, the regulatory mechanisms involved in embryonic development mediated by this receptor are poorly understood. In the present study, we performed transcription activator-like effector nuclease (TALEN)-mediated loss-of-function assays, taking advantage of zebrafish external fertilization, to investigate the function of lrh-1. The digestive organs were affected by lrh-1 depletion as a result of cell-cycle arrest (at the checkpoint of G1 to S phase), but not cell apoptosis. Biochemical analysis revealed that LRH-1 augments the transcriptional activity of β-catenin 1 and 2 via physical interactions. Screening the specific ligand(s) sensed by LRH-1 during organogenesis revealed that phosphatidylcholine (PC), a potential ligand, is the upstream target of LRH-1 during endoderm development. These data provide evidence for the crosstalk between the PC/LRH-1 and Wnt/β-catenin signaling pathways during the expansion growth of endoderm organs.

Inhibition of Cyclooxygenase-2 Alters Wnt/β-Catenin Signaling in the Regenerating Tail of Lizard Hemidactylus flaviviridis

Epimorphic regeneration in vertebrates involves the restoration of lost tissue or organs through the formation of a regeneration blastema and occurs through a complex interaction of a number of molecular signaling pathways. Of the many effectors of successful tail regeneration in the lizard Hemidactylus flaviviridis, one crucial pathway is the cyclooxygenase-2 (COX-2) mediated PGE2 signaling pathway. The current study was aimed at understanding whether COX-2 signaling plays any role in the expression of Wnt/β-Catenin signaling components during regenerative outgrowth in H. flaviviridis. Etoricoxib-selective inhibitor of the inducible isoform of COX-2-was administered to lizards orally. We tested the expression of β-Catenin during wound epidermis and blastema stages in the regenerating tail and found a reduction in its expression in response to drug treatment. Further, it was observed that the expression of canonical Wnt ligands was greatly altered due to COX-2 inhibition. Our results provide evidence of a cross-talk between the COX-2 induced PGE2 pathway and Wnt/β-Catenin signaling in the regenerating lizard tail. An understanding of the interaction among various signaling pathways will help elucidate the mechanism underlying epimorphosis in lizards, the only amniotes capable of appendage regeneration.

Wnt8a expands the pool of embryonic kidney progenitors in zebrafish

During zebrafish embryogenesis the pronephric kidney arises from a small population of posterior mesoderm cells that then undergo expansion during early stages of renal organogenesis. While wnt8 is required for posterior mesoderm formation during gastrulation, it is also transiently expressed in the post-gastrula embryo in the intermediate mesoderm, the precursor to the pronephros and some blood/vascular lineages. Here, we show that knockdown of wnt8a, using a low dose of morpholino that does not disrupt early mesoderm patterning, reduces the number of kidney and blood cells. For the kidney, wnt8a deficiency decreases renal progenitor growth during early somitogenesis, as detected by EdU incorporation, but has no effect on apoptosis. The depletion of the renal progenitor pool in wnt8a knockdown embryos leads to cellular deficits in the pronephros at 24 hpf that are characterised by a shortened distal-most segment and stretched proximal tubule cells. A pulse of the canonical Wnt pathway agonist BIO during early somitogenesis is sufficient to rescue the size of the renal progenitor pool while longer treatment expands the number of kidney cells. Taken together, these observations indicate that Wnt8, in addition to its well-established role in posterior mesoderm patterning, also plays a later role as a factor that expands the renal progenitor pool prior to kidney morphogenesis.

Hyaluronic acid synthesis is required for zebrafish tail fin regeneration

Using genome-wide transcriptional profiling and whole-mount expression analyses of zebrafish larvae, we have identified hyaluronan synthase 3 (has3) as an upregulated gene during caudal fin regeneration. has3 expression is induced in the wound epithelium within hours after tail amputation, and its onset and maintenance requires fibroblast growth factor, phosphoinositide 3-kinase, and transforming growth factor-ß signaling. Inhibition of hyaluronic acid (HA) synthesis by the small molecule 4-methylumbelliferone (4-MU) impairs tail regeneration in zebrafish larvae by preventing injury-induced cell proliferation. In addition, 4-MU reduces the expression of genes associated with wound epithelium and blastema function. Treatment with glycogen synthase kinase 3 inhibitors rescues 4-MU-induced defects in cell proliferation and tail regeneration, while restoring a subset of wound epithelium and blastema markers. Our findings demonstrate a role for HA biosynthesis in zebrafish tail regeneration and delineate its epistatic relationships with other regenerative processes.

Chemical activation of Wnt/β-catenin signalling inhibits innervation and causes skeletal tissue malformations during axolotl limb regeneration

Limb regeneration involves several interrelated physiological processes in which a particular signalling pathway may play a variety of functions. Blocking the function of Wnt/β-catenin signalling during limb regeneration inhibits regeneration in axolotls (Ambystoma mexicanum). Limb development shares many features with limb regeneration, and Wnt/β-catenin activation has different effects depending on the developmental stage. The aim of this study was to evaluate whether Wnt/β-catenin signalling activation during axolotl limb regeneration has different effects when activated at different stages of regeneration. To evaluate this hypothesis, we treated amputated axolotls with a Wnt agonist chemical at different stages of limb regeneration. The results showed that limb regeneration was inhibited when the treatment began before blastema formation. Under these conditions, blastema formation was hindered, possibly due to the lack of innervation. On the other hand, when axolotls were treated after blastema formation and immediately before the onset of morphogenesis, we observed structural disorganization in skeletal formation. In conclusion, we found that limb regeneration was differentially affected depending on the stage at which the Wnt signalling pathway was activated.

Chemical biology reveals CARF as a positive regulator of canonical Wnt signaling by promoting TCF/β-catenin transcriptional activity

Wnt/β-catenin signaling regulates multiple biological processes and aberration of this pathway is frequently observed in human cancers. Previously, we uncovered NC043 as a small-molecule inhibitor of Wnt/β-catenin signaling. Here, we identified CARF as the cellular target of NC043. We found that NC043 binds directly to CARF through forming a covalent bond with the Cys-516 residue of CARF. Further study revealed that CARF interacts with Dvl, which potentiates the Dvl-c-Jun-β-catenin-TCF transcriptional complex and thus promotes Wnt signaling activation. NC043 could disrupt the interaction between CARF and Dvl, thereby impairing Wnt signal transduction. In line with this, knockdown of CARF in zebrafish leads to impairment of embryonic development, hematopoietic stem cell generation and caudal fin regeneration. Collectively, we identified CARF as the cellular target of NC043 and revealed CARF as a positive regulator of Wnt/β-catenin signal transduction.

ptf1a+ , ela3l- cells are developmentally maintained progenitors for exocrine regeneration following extreme loss of acinar cells in zebrafish larvae

The exocrine pancreas displays a significant capacity for regeneration and renewal. In humans and mammalian model systems, the partial loss of exocrine tissue, such as after acute pancreatitis or partial pancreatectomy induces rapid recovery via expansion of surviving acinar cells. In mouse it was further found that an almost complete removal of acinar cells initiates regeneration from a currently not well-defined progenitor pool. Here, we used the zebrafish as an alternative model to study cellular mechanisms of exocrine regeneration following an almost complete removal of acinar cells. We introduced and validated two novel transgenic approaches for genetically encoded conditional cell ablation in the zebrafish, either by caspase-8-induced apoptosis or by rendering cells sensitive to diphtheria toxin. By using the ela3l promoter for exocrine-specific expression, we show that both approaches allowed cell-type-specific removal of >95% of acinar tissue in larval and adult zebrafish without causing any signs of unspecific side effects. We find that zebrafish larvae are able to recover from a virtually complete acinar tissue ablation within 2 weeks. Using short-term lineage-tracing experiments and EdU incorporation assays, we exclude duct-associated Notch-responsive cells as the source of regeneration. Rather, a rare population of slowly dividing ela3l-negative cells expressing ptf1a and CPA was identified as the origin of the newly forming exocrine cells. Cells are actively maintained, as revealed by a constant number of these cells at different larval stages and after repeated cell ablation. These cells establish ela3l expression about 4-6 days after ablation without signs of increased proliferation in between. With onset of ela3l expression, cells initiate rapid proliferation, leading to fast expansion of the ela3l-positive population. Finally, we show that this proliferation is blocked by overexpression of the Wnt-signaling antagonist dkk1b In conclusion, we show a conserved requirement for Wnt signaling in exocrine tissue expansion and reveal a potential novel progenitor or stem cell population as a source for exocrine neogenesis after complete loss of acinar cells.

Proliferation-independent regulation of organ size by Fgf/Notch signaling

Organ morphogenesis depends on the precise orchestration of cell migration, cell shape changes and cell adhesion. We demonstrate that Notch signaling is an integral part of the Wnt and Fgf signaling feedback loop coordinating cell migration and the self-organization of rosette-shaped sensory organs in the zebrafish lateral line system. We show that Notch signaling acts downstream of Fgf signaling to not only inhibit hair cell differentiation but also to induce and maintain stable epithelial rosettes. Ectopic Notch expression causes a significant increase in organ size independently of proliferation and the Hippo pathway. Transplantation and RNASeq analyses revealed that Notch signaling induces apical junctional complex genes that regulate cell adhesion and apical constriction. Our analysis also demonstrates that in the absence of patterning cues normally provided by a Wnt/Fgf signaling system, rosettes still self-organize in the presence of Notch signaling.

DOI:http://dx.doi.org/10.7554/eLife.21049.001