Hedgehog Signaling during Appendage Development and Regeneration

The HH-GLI2-CKS1B network regulates the proliferation-to-maturation transition of cardiomyocytes

Cardiomyocyte (CM) proliferation and maturation are highly linked processes, however, the extent to which these processes are controlled by a single signaling axis is unclear. Here, we show the previously undescribed role of Hedgehog (HH)-GLI2-CKS1B cascade in regulation of the toggle between CM proliferation and maturation. Here we show downregulation of GLI-signaling in adult human CM, adult murine CM, and in late-stage hiPSC-CM leading to their maturation. In early-stage hiPSC-CM, inhibition of HH- or GLI-proteins enhanced CM maturation with increased maturation indices, increased calcium handling, and transcriptome. Mechanistically, we identified CKS1B, as a new effector of GLI2 in CMs. GLI2 binds the CKS1B promoter to regulate its expression. CKS1B overexpression in late-stage hiPSC-CMs led to increased proliferation with loss of maturation in CMs. Next, analysis of datasets of patients with heart disease showed a significant enrichment of GLI2-signaling in patients with ischemic heart failure (HF) or dilated-cardiomyopathy (DCM) disease, indicating operational GLI2-signaling in the stressed heart. Thus, the Hh-GLI2-CKS1B axis regulates the proliferation-maturation transition and provides targets to enhance cardiac tissue engineering and regenerative therapies.

Turning gray selenium into a nanoaccelerator of tissue regeneration by PEG modification

Selenium (Se) is an essential trace element involved in nearly all human physiological processes but suffers from a narrow margin between benefit and toxicity. The nanoform of selenium has been proven shown to be more bioavailable and less toxic, yet significant challenges remain regarding the efficient and feasible synthesis of biologically active nanoselenium. In addition, although nanoselenium has shown a variety of biological activities, more interesting nanoselenium features are expected. In this work, hydrosoluble nanoselenium termed Nano-Se in the zero oxidation state was synthesized between gray Se and PEG. A zebrafish screen was carried out in zebrafish larvae cocultured with Nano-Se. Excitingly, Nano-Se promoted the action of the FGFR, Wnt, and VEGF signaling pathways, which play crucial roles in tissue regeneration. As expected, Nano-Se not only achieved the regeneration of zebrafish tail fins and mouse skin but also promoted the repair of skin in diabetic mice while maintaining a profitable safe profile. In brief, the Nano-Se reported here provided an efficient and feasible method for bioactive nanoselenium synthesis and not only expanded the application of nanoselenium to regenerative medicine but also likely reinvigorated efforts for discovering more peculiarunique biofunctions of nanoselenium in a great variety of human diseases.

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It was found that selenium nanoparticles through FGFR、Wnt、VEGFR signal pathway to promote tissue regeneration;

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Development a new water-soluble, bio-compatible, zero oxidation state Nano-Se;

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Development a new efficient and safe nano-biologic agent for promoting tissue regeneration.

Importance of clitellar tissue in the regeneration ability of earthworm Eudrilus eugeniae

Among the annelids, earthworms are renowned for their phenomenal ability to regenerate the lost segments. The adult earthworm Eudrilus eugeniae contains 120 segments and the body segments of the earthworm are divided into pre-clitellar, clitellar and post-clitellar segments. The present study denoted that clitellum plays vital role in the successful regeneration of the species. We have performed histological studies to identify among the three skin layers of the earthworm, which cellular layer supports the blastema formation and regeneration of the species. The histological evidences denoted that the proliferation of the longitudinal cell layer at the amputation site is crucial for the successful regeneration of the earthworm and it takes place only in the presence of an intact clitellum. Besides we have performed clitellar transcriptome analysis of the earthworm Eudrilus eugeniae to monitor the key differentially expressed genes and their associated functions and pathways controlling the clitellar tissue changes during both anterior and posterior regeneration of the earthworm. A total of 4707 differentially expressed genes (DEGs) were identified between the control clitellum and clitellum of anterior regenerated earthworms and 4343 DEGs were detected between the control clitellum and clitellum of posterior regenerated earthworms. The functional enrichment analysis confirmed the genes regulating the muscle mass shape and structure were significantly downregulated and the genes associated with response to starvation and anterior-posterior axis specification were significantly upregulated in the clitellar tissue during both anterior and posterior regeneration of the earthworm. The RNA sequencing data of clitellum and the comparative transcriptomic analysis were helpful to understand the complex regeneration process of the earthworm.

Self-Control of Inflammation during Tail Regeneration of Lizards

Lizards can spontaneously regenerate their lost tail without evoking excessive inflammation at the damaged site. In contrast, tissue/organ injury of its mammalian counterparts results in wound healing with a formation of a fibrotic scar due to uncontrolled activation of inflammatory responses. Unveiling the mechanism of self-limited inflammation occurring in the regeneration of a lizard tail will provide clues for a therapeutic alternative to tissue injury. The present review provides an overview of aspects of rapid wound healing and roles of antibacterial peptides, effects of leukocytes on the tail regeneration, self-blocking of the inflammatory activation in leukocytes, as well as inflammatory resistance of blastemal cells or immature somatic cells during lizard tail regeneration. These mechanistic insights of self-control of inflammation during lizard tail regeneration may lead in the future to the development of therapeutic strategies to fight injury-induced inflammation.

Non-canonical Hedgehog signaling regulates spinal cord and muscle regeneration in Xenopus laevis larvae

Inducing regeneration in injured spinal cord represents one of modern medicine’s greatest challenges. Research from a variety of model organisms indicates that Hedgehog (Hh) signaling may be a useful target to drive regeneration. However, the mechanisms of Hh signaling-mediated tissue regeneration remain unclear. Here, we examined Hh signaling during post-amputation tail regeneration in Xenopus laevis larvae. We found that while Smoothened (Smo) activity is essential for proper spinal cord and skeletal muscle regeneration, transcriptional activity of the canonical Hh effector Gli is repressed immediately following amputation, and inhibition of Gli1/2 expression or transcriptional activity has minimal effects on regeneration. In contrast, we demonstrate that protein kinase A is necessary for regeneration of both muscle and spinal cord, in concert with and independent of Smo, respectively, and that its downstream effector CREB is activated in spinal cord following amputation in a Smo-dependent manner. Our findings indicate that non-canonical mechanisms of Hh signaling are necessary for spinal cord and muscle regeneration.

Regenerative engineering: a review of recent advances and future directions

Regenerative engineering is defined as the convergence of the disciplines of advanced material science, stem cell science, physics, developmental biology and clinical translation for the regeneration of complex tissues and organ systems. It is an expansion of tissue engineering, which was first developed as a method of repair and restoration of human tissue. In the past three decades, advances in regenerative engineering have made it possible to treat a variety of clinical challenges by utilizing cutting-edge technology currently available to harness the body's healing and regenerative abilities. The emergence of new information in developmental biology, stem cell science, advanced material science and nanotechnology have provided promising concepts and approaches to regenerate complex tissues and structures.

Hedgehog signaling regulates regenerative patterning and growth in Harmonia axyridis leg

Appendage regeneration has been widely studied in many species. Compared to other animal models, Harmonia axyridis has the advantage of a short life cycle, is easily reared, has strong regeneration capacity and contains systemic RNAi, making it a model organism for research on appendage regeneration. Here, we performed transcriptome analysis, followed by gene functional assays to reveal the molecular mechanism of H. axyridis leg regenerative growth process. Signaling pathways including Decapentaplegic (Dpp), Wingless (Wg), Ds/Ft/Hippo, Notch, Egfr, and Hedgehog (Hh) were all upregulated during the leg regenerative patterning and growth. Among these, Hh and its auxiliary receptor Lrp2 were required for the proper patterning and growth of the regenerative leg. The targets of canonical Hh signaling were required for the regenerative growth which contributes to the leg length, but were not essential for the pattern formation of the regenerative leg. dpp, wg and leg developmental-related genes including rn, dac and Dll were all regulated by hh and lrp2 and may play an essential role in the regenerative patterning of the leg.

Exploring the role of microRNAs in axolotl regeneration

The axolotl, Ambystoma mexicanum, is used extensively for research in developmental biology, particularly for its ability to regenerate and restore lost organs, including in the nervous system, to full functionality. Regeneration in mammals typically depends on the healing process and scar formation with limited replacement of lost tissue. Other organisms, such as spiny mice (Acomys cahirinus), salamanders, and zebrafish, are able to regenerate some damaged body components. Blastema is a tissue that is formed after tissue injury in such organisms and is composed of progenitor cells or dedifferentiated cells that differentiate into various cell types during regeneration. Thus, identifying the molecules responsible for initiation of blastema formation is an important aspect for understanding regeneration. Introns, a major source of noncoding RNAs (ncRNAs), have characteristic sizes in the axolotl, particularly in genes associated with development. These ncRNAs, particularly microRNAs (miRNAs), exhibit dynamic regulation during regeneration. These miRNAs play an essential role in timing and control of gene expression to order and organize processes necessary for blastema creation. Master keys or molecules that underlie the remarkable regenerative abilities of the axolotl remain to be fully explored and exploited. Further and ongoing research on regeneration promises new knowledge that may allow improved repair and renewal of human tissues.

Harnessing hypoxia as an evolutionary driver of complex multicellularity

Animal tissue requires low-oxygen conditions for its maintenance. The need for low-oxygen conditions contrasts with the idea of an evolutionary leap in animal diversity as a result of expanding oxic conditions. To accommodate tissue renewal at oxic conditions, however, vertebrate animals and vascular plants demonstrate abilities to access hypoxia. Here, I argue that multicellular organisms sustain oxic conditions first after internalizing hypoxic conditions. The 'harnessing' of hypoxia has allowed multicellular evolution to leave niches that were stable in terms of oxygen concentrations for those where oxygen fluctuates. Since oxygen fluctuates in most settings on Earth's surface, the ancestral niche would have been a deep marine setting. The hypothesis that 'large life' depends on harnessing hypoxia is illustrated in the context of conditions that promote the immature cell phenotype (stemness) in animal physiology and tumour biology and offers one explanation for the general rarity of diverse multicellularity over most of Earth's history.

Hedgehog Signal Inhibitor GANT61 Inhibits the Malignant Behavior of Undifferentiated Hepatocellular Carcinoma Cells by Targeting Non-Canonical GLI Signaling

The Hedgehog (HH)–GLI pathway plays an important role in cell dedifferentiation and is therefore pivotally involved in the malignant transformation of cancer cells. GANT61, a selective inhibitor of GLI1 and GLI2, was reported as a promising treatment for cancer in various tissues; however, the biological impact of GANT61 in hepatocellular carcinoma (HCC), especially in undifferentiated HCC cells, remains unclear. In this study, we investigated the antitumor effect of GANT61 using two undifferentiated hepatoma cell lines: HLE and HLF. Quantitative PCR and RT-PCR analyses revealed that these cells express GLI transcripts, showing mesenchymal phenotypes characterized by the loss of epithelial and hepatic markers and specific expression of epithelial–mesenchymal transition (EMT)-related genes. GANT61 significantly reduced the proliferation and cell viability after drug treatment using 5-FU and Mitomycin C. We showed that GLI transcript levels were down-regulated by the MEK inhibitor U0126 and the Raf inhibitor sorafenib, suggesting that non-canonical signaling including the Ras–Raf–MEK–ERK pathway is involved. Sphere formation and migration were significantly decreased by GANT61 treatment, and it is suggested that the underlying molecular mechanisms are the down-regulation of stemness-related genes (Oct4, Bmi1, CD44, and ALDH) and the EMT-related gene Snail1. The data presented here showed that direct inhibition of GLI might be beneficial for the treatment of dedifferentiated HCC.

Endocrine Regulation of Epimorphic Regeneration

Studies aiming to uncover primary mechanisms of regeneration have predominantly focused on genetic pathways regulating specific stages in the regeneration process: wound healing, blastema formation, and pattern formation. However, studies across organisms show that environmental conditions and the physiological state of the animal can affect the rate or quality of regeneration, and endocrine signals are likely the mediators of these effects. Endocrine signals acting directly on receptors expressed in the tissue or via neuroendocrine pathways can affect regeneration by regulating the immune response to injury, allocation of energetic resources, or by enhancing or inhibiting proliferation and differentiation pathways involved in regeneration. This review discusses the cumulative knowledge in the literature about endocrine regulation of regeneration and its importance in future research to advance biomedical research.

Endocrine regulation of regeneration: Linking global signals to local processes

Regeneration in amphibians and reptiles has been explored since the early 18th century, giving us a working in vivo model to study epimorphic regeneration in vertebrates. Studies aiming to uncover primary mechanisms of regeneration have predominantly focused on genetic pathways regulating specific stages of the regeneration process: wound healing, blastema formation and growth, and pattern formation. However, studies across organisms show that environmental conditions and physiological state of the animal can affect the rate or quality of regeneration, and endocrine signals are likely the mediators of these effects. Endocrine signals working/acting directly on receptors expressed in the structure or via neuroendocrine pathways can affect regeneration by modulating immune response to injury, allocation of energetic resources, or by enhancing or inhibiting proliferation and differentiation pathways in regenerating tissue. This review discusses the cumulative knowledge known about endocrine regulation of regeneration and important future research directions of interest to both ecological and biomedical research.

Electrical stimulation shifts healing/scarring towards regeneration in a rat limb amputation model

Different species respond differently to severe injury, such as limb loss. In species that regenerate, limb loss is met with complete restoration of the limbs’ form and function, whereas in mammals the amputated limb’s stump heals and scars. In in vitro studies, electrical stimulation (EStim) has been shown to promote cell migration, and osteo- and chondrogenesis. In in vivo studies, after limb amputation, EStim causes significant new bone, cartilage and vessel growth. Here, in a rat model, the stumps of amputated rat limbs were exposed to EStim, and we measured extracellular matrix (ECM) deposition, macrophage distribution, cell proliferation and gene expression changes at early (3 and 7 days) and later stages (28 days). We found that EStim caused differences in ECM deposition, with less condensed collagen fibrils, and modified macrophage response by changing M1 to M2 macrophage ratio. The number of proliferating cells was increased in EStim treated stumps 7 days after amputation, and transcriptome data strongly supported our histological findings, with activated gene pathways known to play key roles in embryonic development and regeneration. In conclusion, our findings support the hypothesis that EStim shifts injury response from healing/scarring towards regeneration. A better understanding of if and how EStim controls these changes, could lead to strategies that replace scarring with regeneration.

A multi-institutional study benchmarking the zebrafish developmental assay for prediction of embryotoxic plasma concentrations from rat embryo-fetal development studies

Predicting embryotoxicity of pharmaceutical compounds or industrial chemicals is crucial for public safety. Conventional studies which monitor embryo-fetal development in rats and rabbits are costly and time consuming. Alternative assays which are simpler and less costly are being pursued. The purpose of this research was to assess the capacity for the zebrafish development assay to predict mammalian plasma levels that are embryotoxic. Previously published data on rat plasma levels associated with embryotoxicity were used to guide concentration ranges for each of 25 chemicals dissolved in the media bathing developing zebrafish embryos. Embryotoxic media concentrations were compared to embryotoxic rat plasma concentrations. Assays were conducted in parallel at multiple sites as a consortium effort through the Health and Environmental Sciences Institute (HESI). Considering results from all sites, the zebrafish embryo development assay predicted (within 1-log) the rat maternal exposure levels associated with embryotoxicity 75% of the time.

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.

A conserved HH-Gli1-Mycn network regulates heart regeneration from newt to human

The mammalian heart has a limited regenerative capacity and typically progresses to heart failure following injury. Here, we defined a hedgehog (HH)-Gli1-Mycn network for cardiomyocyte proliferation and heart regeneration from amphibians to mammals. Using a genome-wide screen, we verified that HH signaling was essential for heart regeneration in the injured newt. Next, pharmacological and genetic loss- and gain-of-function of HH signaling demonstrated the essential requirement for HH signaling in the neonatal, adolescent, and adult mouse heart regeneration, and in the proliferation of hiPSC-derived cardiomyocytes. Fate-mapping and molecular biological studies revealed that HH signaling, via a HH-Gli1-Mycn network, contributed to heart regeneration by inducing proliferation of pre-existing cardiomyocytes and not by de novo cardiomyogenesis. Further, Mycn mRNA transfection experiments recapitulated the effects of HH signaling and promoted adult cardiomyocyte proliferation. These studies defined an evolutionarily conserved function of HH signaling that may serve as a platform for human regenerative therapies.

Due to the limited proliferation capacity of adult mammalian cardiomyocytes, the human heart has negligible regenerative capacity after injury. Here the authors show that a Hedgehog-Gli1-Mycn signaling cascade regulates cardiomyocyte proliferation and cardiac regeneration from amphibians to mammals.

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.

TSC1 and TSC2 regulate cilia length and canonical Hedgehog signaling via different mechanisms

Primary cilia are sensory organelles that coordinate multiple cellular signaling pathways, including Hedgehog (HH), Wingless/Int (WNT) and Transforming Growth Factor-β (TGF-β) signaling. Similarly, primary cilia have been implicated in regulation of mTOR signaling, in which Tuberous Sclerosis Complex proteins 1 and 2 (TSC1/2) negatively regulate protein synthesis by inactivating the mTOR complex 1 (mTORC1) at energy limiting states. Here we report that TSC1 and TSC2 regulate Smoothened (SMO)-dependent HH signaling in mouse embryonic fibroblasts (MEFs). Reduced SMO-dependent expression of Gli1 was demonstrated in both Tsc1-/- and Tsc2-/- cells, and we found that Tsc1 is required for TGF-β induced phosphorylation of SMAD2/3 and subsequent expression of the HH signaling effector and transcription factor GLI2. Hedgehog signaling was restored in Tsc1-/- cells after exogenous expression of Gli2, whereas rapamycin restored HH signaling in Tsc2-/- cells. Furthermore, we observed that Tsc1-/- MEFs display significantly elongated cilia, whereas cilia in Tsc2-/- MEFs were shorter than normal. The elongated cilium phenotype of Tsc1-/- MEFs is likely due to increased mTORC1-dependent autophagic flux observed in these cells, as both the autophagic flux and the cilia length phenotype was restored by rapamycin. In addition, ciliary length control in Tsc1-/- MEFs was also influenced by reduced expression of Gli2, which compromised expression of Wnt5a that normally promotes cilia disassembly. In summary, our results support distinct functions of Tsc1 and Tsc2 in cellular signaling as the two genes affect ciliary length control and HH signaling via different mechanisms.

Etv2-miR-130a-Jarid2 cascade regulates vascular patterning during embryogenesis

Remodeling of the primitive vasculature is necessary for the formation of a complex branched vascular architecture. However, the factors that modulate these processes are incompletely defined. Previously, we defined the role of microRNAs (miRNAs) in endothelial specification. In the present study, we further examined the Etv2-Cre mediated ablation of Dicer^L/L and characterized the perturbed vascular patterning in the embryo proper and yolk-sac. We mechanistically defined an important role for miR-130a, an Etv2 downstream target, in the mediation of vascular patterning and angiogenesis in vitro and in vivo. Inducible overexpression of miR-130a resulted in robust induction of vascular sprouts and angiogenesis with increased uptake of acetylated-LDL. Mechanistically, miR-130a directly regulated Jarid2 expression by binding to its 3’-UTR region. Over-expression of Jarid2 in HUVEC cells led to defective tube formation indicating its inhibitory role in angiogenesis. The knockout of miR-130a showed increased levels of Jarid2 in the ES/EB system. In addition, the levels of Jarid2 transcripts were increased in the Etv2-null embryos at E8.5. In the in vivo settings, injection of miR-130a specific morpholinos in zebrafish embryos resulted in perturbed vascular patterning with reduced levels of endothelial transcripts in the miR-130a morphants. Further, co-injection of miR-130a mimics in the miR-130a morphants rescued the vascular defects during embryogenesis. qPCR and in situ hybridization techniques demonstrated increased expression of jarid2a in the miR-130a morphants in vivo. These findings demonstrate a critical role for Etv2-miR-130a-Jarid2 in vascular patterning both in vitro and in vivo.

Nerves, H2O2 and Shh: Three players in the game of regeneration

The tight control of reactive oxygen species (ROS) levels is required during regeneration. H₂O₂ in particular assumes clear signalling functions at different steps in this process. Injured nerves induce high levels of H₂O₂ through the activation of the Hedgehog (Shh) pathway, providing an environment that promotes cell plasticity, progenitor recruitment and blastema formation. In turn, high H₂O₂ levels contribute to growing axon attraction. Once re-innervation is completed, nerves subsequently downregulate H₂O₂ levels to their original state. A similar regulatory loop between H₂O₂ levels and nerves also exists during development. This suggests that redox signalling is a major actor in cell plasticity.

Dpath software reveals hierarchical haemato-endothelial lineages of Etv2 progenitors based on single-cell transcriptome analysis

Developmental, stem cell and cancer biologists are interested in the molecular definition of cellular differentiation. Although single-cell RNA sequencing represents a transformational advance for global gene analyses, novel obstacles have emerged, including the computational management of dropout events, the reconstruction of biological pathways and the isolation of target cell populations. We develop an algorithm named dpath that applies the concept of metagene entropy and allows the ranking of cells based on their differentiation potential. We also develop self-organizing map (SOM) and random walk with restart (RWR) algorithms to separate the progenitors from the differentiated cells and reconstruct the lineage hierarchies in an unbiased manner. We test these algorithms using single cells from Etv2-EYFP transgenic mouse embryos and reveal specific molecular pathways that direct differentiation programmes involving the haemato-endothelial lineages. This software program quantitatively assesses the progenitor and committed states in single-cell RNA-seq data sets in a non-biased manner.

Single-cell RNA sequencing has enabled great advances in understanding developmental biology but reconstructing cellular lineages from this data remains challenging. Here the authors develop an algorithm, dpath, which models the lineage relationships of underlying single cells based on single cell RNA seq data and apply it to study lineage progression of Etv2 expressing progenitors.

The Quest toward limb regeneration: a regenerative engineering approach

The Holy Grail to address the clinical grand challenge of human limb loss is to develop innovative strategies to regrow the amputated limb. The remarkable advances in the scientific understanding of regeneration, stem cell science, material science and engineering, physics and novel surgical approaches in the past few decades have provided a regenerative tool box to face this grand challenge and address the limitations of human wound healing. Here we discuss the convergence approach put forward by the field of Regenerative Engineering to use the regenerative tool box to design and develop novel translational strategies to limb regeneration.

Trop2 marks transient gastric fetal epithelium and adult regenerating cells after epithelial damage

Mouse fetal intestinal progenitors lining the epithelium prior to villogenesis grow as spheroids when cultured ex vivo and express the transmembrane glycoprotein Trop2 as a marker. Here, we report the characterization of Trop2-expressing cells from fetal pre-glandular stomach, growing as immortal undifferentiated spheroids, and their relationship with gastric development and regeneration. Trop2⁺ cells generating gastric spheroids differed from adult glandular Lgr5⁺ stem cells, but appeared highly related to fetal intestinal spheroids. Although they shared a common spheroid signature, intestinal and gastric fetal spheroid-generating cells expressed organ-specific transcription factors and were committed to intestinal and glandular gastric differentiation, respectively. Trop2 expression was transient during glandular stomach development, being lost at the onset of gland formation, whereas it persisted in the squamous forestomach. Undetectable under homeostasis, Trop2 was strongly re-expressed in glands after acute Lgr5⁺ stem cell ablation or following indomethacin-induced injury. These highly proliferative reactive adult Trop2⁺ cells exhibited a transcriptome displaying similarity with that of gastric embryonic Trop2⁺ cells, suggesting that epithelium regeneration in adult stomach glands involves the partial re-expression of a fetal genetic program.

Summary: Trop2, a marker of gastric fetal glandular epithelium grown ex vivo, is re-expressed upon injury in adult regenerative cells together with a partial fetal-like genetic program.